mode of ipv4 address assignment not set

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Configuration validation

Home > Networking Solutions > Modular/Blade Switches > MX-Series Modular Switches (PowerEdge MX) > Guides > Dell PowerEdge MX Networking Deployment Guide > Configuration validation

Show Eth-npar

The following show command details the NPAR interfaces.

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[SOLVED]   Router not assigning IPv4

• Thread starter pmb88
• Start date May 25, 2020

Distinguished

• May 25, 2020

I have an issue reoccurring. I am using a Linksys E2500 router on Win 10. Lately on my PC some sites work like Facebook and Google, but can't access others. I tried to access router page using the default gateway 192.168.1.1, but it doesn't work. I noticed that on my network adapter has an IPv6 address, but no IPv4. I have to restart the router and modem several times. I had this router for several years, and this issue just started recently. I haven't done any changes to the router. Is the router reaching it's end?  

• Jun 10, 2020

Your current router only has 100mbps ports so that is what is causing the 92mbps speedtest. Look for a router that has gigabit ports. You should be able to get something fairly good for $50-$60 range. Stay with the better brand routers. Even very inexpensive routers can do 1gbit wan/lan on ethernet. Wifi is going to be more limited by your end devices so don't be conned by big numbers. Lately almost all routers with number between 1200-1750 are only a few dollars different in price. You likely do not need anything with a bigger number.

pmb88 said: I have an issue reoccurring. I am using a Linksys E2500 router on Win 10. Lately on my PC some sites work like Facebook and Google, but can't access others. I tried to access router page using the default gateway 192.168.1.1, but it doesn't work. I noticed that on my network adapter has an IPv6 address, but no IPv4. I have to restart the router and modem several times. I had this router for several years, and this issue just started recently. I haven't done any changes to the router. Is the router reaching it's end? Click to expand...

• 2001:4860:4860::8888
• 2001:4860:4860::8844

Illustrious

Sounds like either 1) Time to get a new one or 2) Factory reset it and see if you can get it going again that way.  

smithadamtries

• Disable IPV6 (Sometimes it interferes)
• Setup a Static IPV4 (try it with Static DNS.)
• Factory reset (It might not solve it if it's not a router settings problem, but worth trying!)
• May 26, 2020

Since nothing has changed in your configuration, I would book a linux live cd/usb to rule out any OS or hardware problems. If it doesn't work on the live cd, then I'd suspect hardware or network problems. I would also try wifi to see if that is any different. If wifi works and ethernet doesn't in linux, that points to a ethernet port going bad.  

• May 29, 2020
• Assigning static ip - this didn't appear to work.
• Factory reset - it seemed to work nicely for a couple of days. However, the issue reoccurred yesterday.

pmb88 said: When the issue occurs, it occurs on any device whether they are connected via wifi or ethernet. Here's what I've tried since my post: Click to expand...

• May 30, 2020

It happened again yesterday. I directly connected to the modem and had no problems. So it seems to be something with the router.  

• May 31, 2020

pmb88 said: It happened again yesterday. I directly connected to the modem and had no problems. So it seems to be something with the router. Click to expand...

• Jun 2, 2020

Since it has been determined that it is the router, then what would be a suitable replacement?  

pmb88 said: Since it has been determined that it is the router, then what would be a suitable replacement? Click to expand...

SamirD said: How fast is your plan? Click to expand...

• Jun 7, 2020

pmb88 said: It's about 92 Mbps download, 25 Mbps upload Click to expand...

SamirD said: Is that a speed test or what you are paying for? Click to expand...

Your current router only has 100mbps ports so that is what is causing the 92mbps speedtest. Look for a router that has gigabit ports. You should be able to get something fairly good for $50-$60 range. Stay with the better brand routers. Even very inexpensive routers can do 1gbit wan/lan on ethernet. Wifi is going to be more limited by your end devices so don't be conned by big numbers. Lately almost all routers with number between 1200-1750 are only a few dollars different in price. You likely do not need anything with a bigger number.  

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Static IP won't stay assigned

I have a Windows 7 desktop (64-bit) that has both a wireless and wired adapter. I have been able to successfully assign the wireless with a static IP address of 192.168.1.169 but when I try to assign the wired adapter to 192.168.1.150, and choose to have it validate on close, it wants to fix the DHCP, which in turn removes the static info I put in.

If I tell it not to validate on close, it will not work at all, it says Unidentified Network .

How can I disable the DHCP? I tried assigning it, not validating, then power cycling the computer and router, but no luck.

Any help you can give would be greatly appreciated.

• 1 I experienced this a few days back, i was able to solve it after some searching. This should help < support.microsoft.com/en-us/help/937056/… > –  user774980 Sep 26, 2017 at 16:13

2 Answers 2

How can I disable the DHCP?

From the 'Network and sharing center':

• Left click on the NIC
• Select 'properties'.
• Select 'Internet protocol version 4 (TCP/IP v4)
• Change the point from 'Obtain an IP address automatically` to 'Use the following IP adress'.

Since you already tried setting a manual IP you already been to this place.

You need to fill in at least two fields: 'ip address' and Subnet mask. (One without the other is useless). Did you enter values in both of these before trying to exit?

The remaining fields are the default gateway and the DNS servers.

You do not need to set a default gateway, but if you do not set the default gateway then you will not be able to reach the internet, or any other machines not on your local subnet.

Similarly you do not need to set DNS servers. Not setting a DNS server means you will have to type IP addresses rather then hostname.

I recommend looking at the current values of both of these (and the netmask), and then returning to this screen and trying to set the IP again. (With 'Validate settings upon exit` ticked).

If you do not know what all these things mean, then I recommend taking a look at how-does-ipv4-subnetting-work .

I too have had a similar problem. Every time I set the static IP it would change back to DHCP. I found and corrected the problem. I have a Lenovo laptop that is running all the Thinkpad tools, Access Connections to be precise. This network interface manager takes precedence over Microsoft and I only needed to create a static IP profile in Access Connections and all is good again.

I imagine there are other network configuration tools/aids out there that probably act the same way. Access Connections is really very nice, I just never made the connection that it would prevent changing the network interfaces thru the OS panels.

You must log in to answer this question.

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IP Addressing: IPv4 Addressing Configuration Guide, Cisco IOS XE Release 3S

Bias-free language.

The documentation set for this product strives to use bias-free language. For the purposes of this documentation set, bias-free is defined as language that does not imply discrimination based on age, disability, gender, racial identity, ethnic identity, sexual orientation, socioeconomic status, and intersectionality. Exceptions may be present in the documentation due to language that is hardcoded in the user interfaces of the product software, language used based on RFP documentation, or language that is used by a referenced third-party product. Learn more about how Cisco is using Inclusive Language.

Configuring IPv4 Addresses

• IP Overlapping Address Pools
• IP Unnumbered Ethernet Polling Support
• Zero Touch Auto-IP

Chapter: Configuring IPv4 Addresses

Finding feature information, information about ip addresses, binary numbering, ip address structure, ip address classes, ip network subnetting, ip network address assignments, classless inter-domain routing, how to configure ip addresses, establishing ip connectivity to a network by assigning an ip address to an interface, troubleshooting tips, increasing the number of ip hosts that are supported on a network by using secondary ip addresses, what to do next, maximizing the number of available ip subnets by allowing the use of ip subnet zero, specifying the format of network masks, specifying the format in which netmasks appear for the current session, specifying the format in which netmasks appear for an individual line, using ip unnumbered interfaces on point-to-point wan interfaces to limit number of ip addresses required, ip unnumbered feature, using ip addresses with 31-bit prefixes on point-to-point wan interfaces to limit number of ip addresses required, configuration examples for ip addresses, example establishing ip connectivity to a network by assigning an ip address to an interface, example increasing the number of ip hosts that are supported on a network by using secondary ip addresses, example using ip unnumbered interfaces on point-to-point wan interfaces to limit number of ip addresses required, example using ip addresses with 31-bit prefixes on point-to-point wan interfaces to limit number of ip addresses required, example maximizing the number of available ip subnets by allowing the use of ip subnet zero, where to go next, additional references, feature information for ip addresses.

This chapter contains information about, and instructions for configuring IPv4 addresses on interfaces that are part of a networking device.

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn . An account on Cisco.com is not required.

IP addresses are 32 bits long. The 32 bits are divided into four octets (8-bits). A basic understanding of binary numbering is very helpful if you are going to manage IP addresses in a network because changes in the values of the 32 bits indicate either a different IP network address or IP host address.

A value in binary is represented by the number (0 or 1) in each position multiplied by the number 2 to the power of the position of the number in sequence, starting with 0 and increasing to 7, working right to left. The figure below is an example of an 8-digit binary number.

The figure below provides binary to decimal number conversion for 0 through 134.

The figure below provides binary to decimal number conversion for 135 through 255.

An IP host address identifies a device to which IP packets can be sent. An IP network address identifies a specific network segment to which one or more hosts can be connected. The following are characteristics of IP addresses:

IP addresses are 32 bits long

IP addresses are divided into four sections of one byte (octet) each

IP addresses are typically written in a format known as dotted decimal

The table below shows some examples of IP addresses.

IP addresses are further subdivided into two sections known as network and host. The division is accomplished by arbitrarily ranges of IP addresses to classes. For more information see RFC 791 Internet Protocol at http:/​/​www.ietf.org/​rfc/​rfc0791.txt .

In order to provide some structure to the way IP addresses are assigned, IP addresses are grouped into classes. Each class has a range of IP addresses. The range of IP addresses in each class is determined by the number of bits allocated to the network section of the 32-bit IP address. The number of bits allocated to the network section is represented by a mask written in dotted decimal or with the abbreviation / n where n = the numbers of bits in the mask.

The table below lists ranges of IP addresses by class and the masks associated with each class. The digits in bold indicate the network section of the IP address for each class. The remaining digits are available for host IP addresses. For example, IP address 10.90.45.1 with a mask of 255.0.0.0 is broken down into a network IP address of 10.0.0.0 and a host IP address of 0.90.45.1.

When a digit that falls within the network mask changes from 1 to 0 or 0 to 1 the network address is changed. For example, if you change 10101100.00010000.01011001.00100010/16 to 10101100.00110000.01011001.00100010/16 you have changed the network address from 172.16.89.34/16 to 172.48.89.34/16.

When a digit that falls outside the network mask changes from 1 to 0 or 0 to 1 the host address is changed. For example, if you change 10101100.00010000.01011001.00100010/16 to 10101100.00010000.01011001.00100011/16 you have changed the host address from 172.16.89.34/16 to 172.16.89.35/16.

Each class of IP address supports a specific range of IP network addresses and IP host addresses. The range of IP network addresses available for each class is determined with the formula 2 to the power of the number of available bits. In the case of class A addresses, the value of the first bit in the 1st octet (as shown in the table above) is fixed at 0. This leaves 7 bits for creating additional network addresses. Therefore there are 128 IP network addresses available for class A (27 = 128).

The number of IP host addresses available for an IP address class is determined by the formula 2 to the power of the number of available bits minus 2. There are 24 bits available in a class A addresses for IP host addresses. Therefore there are 16,777,214 IP hosts addresses available for class A ((224) - 2 = 16,777,214)).

The table below shows the network and host addresses available for each class of IP address.

The arbitrary subdivision of network and host bits in IP address classes resulted in an inefficient allocation of IP space. For example, if your network has 16 separate physical segments you will need 16 IP network addresses. If you use 16 class B IP network addresses, you would be able to support 65,534 hosts on each of the physical segments. Your total number of supported host IP addresses is 1,048,544 (16 * 65,534 = 1,048,544). Very few network technologies can scale to having 65,534 hosts on a single network segment. Very few companies need 1,048,544 IP host addresses. This problem required the development of a new strategy that permitted the subdivision of IP network addresses into smaller groupings of IP subnetwork addresses. This strategy is known as subnetting.

If your network has 16 separate physical segments you will need 16 IP subnetwork addresses. This can be accomplished with one class B IP address. For example, start with the class B IP address of 172.16.0.0 you can reserve 4 bits from the third octet as subnet bits. This gives you 16 subnet IP addresses 24 = 16. The table below shows the IP subnets for 172.16.0.0/20.

When a digit that falls within the subnetwork (subnet) mask changes from 1 to 0 or 0 to 1 the subnetwork address is changed. For example, if you change 10101100.00010000.01011001.00100010/20 to 10101100.00010000.01111001.00100010/20 you have changed the network address from 172.16.89.34/20 to 172.16.121.34/20.

When a digit that falls outside the subnet mask changes from 1 to 0 or 0 to 1 the host address is changed. For example, if you change 10101100.00010000.01011001.00100010/20 to 10101100.00010000.01011001.00100011/20 you have changed the host address from 172.16.89.34/20 to 172.16.89.35/20.

Some people get confused about the terms network address and subnet or subnetwork addresses and when to use them. In the most general sense the term network address means “the IP address that routers use to route traffic to a specific network segment so that the intended destination IP host on that segment can receive it”. Therefore the term network address can apply to both non-subnetted and subnetted IP network addresses. When you are troubleshooting problems with forwarding traffic from a router to a specific IP network address that is actually a subnetted network address, it can help to be more specific by referring to the destination network address as a subnet network address because some routing protocols handle advertising subnet network routes differently from network routes. For example, the default behavior for RIP v2 is to automatically summarize the subnet network addresses that it is connected to their non-subnetted network addresses (172.16.32.0/24 is advertised by RIP v2 as 172.16.0.0/16) when sending routing updates to other routers. Therefore the other routers might have knowledge of the IP network addresses in the network, but not the subnetted network addresses of the IP network addresses.

Routers keep track of IP network addresses to understand the network IP topology (layer 3 of the OSI reference model) of the network to ensure that IP traffic can be routed properly. In order for the routers to understand the network layer (IP) topology, every individual physical network segment that is separated from any other physical network segment by a router must have a unique IP network address.

The figure below shows an example of a simple network with correctly configured IP network addresses. The routing table in R1 looks like the table below.

The figure below shows an example of a simple network with incorrectly configured IP network addresses. The routing table in R1 looks like the table below. If the PC with IP address 172.31.32.3 attempts to send IP traffic to the PC with IP address 172.31.32.54, router R1 cannot determine which interface that the PC with IP address 172.31.32.54 is connected to.

To help prevent mistakes as shown in the figure above, Cisco IOS-based networking devices will not allow you to configure the same IP network address on two or more interfaces in the router when IP routing is enabled.

The only way to prevent the mistake shown in the figure below, where 172.16.31.0/24 is used in R2 and R3, is to have very accurate network documentation that shows where you have assigned IP network addresses.

For a more thorough explanation of IP routing, see the "Related Documents" section for a list of documents related to IP routing.

Due to the continuing increase in internet use and the limitations on how IP addresses can be assigned using the class structure shown in the table above, a more flexible method for allocating IP addresses was required. The new method is documented in RFC 1519 Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy . CIDR allows network administrators to apply arbitrary masks to IP addresses to create an IP addressing plan that meets the requirements of the networks that they administrate.

For more information on CIDR, refer to RFC 1519 at http:/​/​www.ietf.org/​rfc/​rfc1519.txt .

The term prefix is often used to refer to the number of bits of an IP network address that are of importance for building routing tables. If you are using only classful (strict adherence to A, B, and C network address boundaries) IP addresses, the prefixes are the same as the masks for the classes of addresses. For example, using classful IP addressing, a class C IP network address such as 192.168.10.0 uses a 24-bit mask (/24 or 255.255.255.0) and can also be said to have a 24-bit prefix.

If you are using CIDR, the prefixes are arbitrarily assigned to IP network addresses based on how you want to populate the routing tables in your network. For example, a group of class C IP addresses such as 192.168.10.0, 192.168.11.0, 192.168.12.0, 192.168.13.0 can be advertised as a single route to 192.168.0.0 with a 16-bit prefix (192.168.0.0/16). This results in a 4:1 reduction in the number of routes that the routers in your network need to manage.

Perform this task to configure an IP address on an interface.

1.     enable

2.     configure terminal

3.     interface type number

4.     no shutdown

5.     ip address ip-address mask

The following commands can help troubleshoot IP addressing:

show ip interface --Displays the IP parameters for the interface.

show ip route connected --Displays the IP networks the networking device is connected to.

If you have a situation in which you need to connect more IP hosts to a network segment and you have used all of the available IP host addresses for the subnet to which you have assigned the segment, you can avoid having to readdress all of the hosts with a different subnet by adding a second IP network address to the network segment.

Perform this task to configure a secondary IP address on an interface.

6.     ip address ip-address mask secondary

If your network has two or more routers and you have already configured a routing protocol, make certain that the other routers can reach the new IP network that you assigned. You might need to modify the configuration for the routing protocol on the router so that it advertises the new network. Consult the Cisco IOS IP Routing: Protocol-Independent Configuration Guide for information on configuring routing protocols.

If you using subnetting in your network and you are running out of network addresses, you can configure your networking device to allow the configuration of subnet zero. This adds one more usable network address for every subnet in your IP addressing scheme. The table above shows the IP subnets (including subnet 0) for 172.16.0.0/20.

Perform this task to enable the use of IP subnet zero on your networking device.

3.     ip subnet-zero

4.     interface type number

5.     no shutdown

6.     ip address ip-address mask

By default, show commands display an IP address and then its netmask in dotted decimal notation. For example, a subnet would be displayed as 131.108.11.55 255.255.255.0.

You might find it more convenient to display the network mask in hexadecimal format or bit count format instead. The hexadecimal format is commonly used on UNIX systems. The previous example would be displayed as 131.108.11.55 0XFFFFFF00.

The bit count format for displaying network masks is to append a slash (/) and the total number of bits in the netmask to the address itself. The previous example would be displayed as 131.108.11.55/24.

Perform this task to specify the format in which netmasks appear for the current session.

2.     term ip netmask-format { bitcount | decimal | hexadecimal }

Perform this task to specify the format in which netmasks appear for an individual line.

3.     line vty first last

4.     term ip netmask-format { bitcount | decimal | hexadecimal }

If you have a limited number of IP network or subnet addresses and you have point-to-point WANs in your network, you can use the IP Unnumbered Interfaces feature to enable IP connectivity on the point-to-point WAN interfaces without actually assigning an IP address to them.

Perform this task to configure the IP Unnumbered Interfaces feature on a point-to-point WAN interface.

The IP Unnumbered Interfaces feature enables IP processing on a point-to-point WAN interface without assigning it an explicit IP address. The IP unnumbered point-to-point WAN interface uses the IP address of another interface to enable IP connectivity, which conserves network addresses.

6.     interface type number

7.     no shutdown

8.     ip unnumbered type number

You can reduce the number of IP subnets used by networking devices to establish IP connectivity to point-to-point WANs that they are connected to by using IP Addresses with 31-bit Prefixes as defined in RFC 3021.

Perform this task to configure an IP address with a 31-bit prefix on a point-to-point WAN interface.

Prior to RFC 3021, Using 31-bit Prefixes on IPv4 Point-to-Point Links , many network administrators assigned IP address with a 30-bit subnet mask (255.255.255.252) to point-to-point interfaces to conserve IP address space. Although this practice does conserve IP address space compared to assigning IP addresses with shorter subnet masks such as 255.255.255.240, IP addresses with a 30-bit subnet mask still require four addresses per link: two host addresses (one for each host interface on the link), one all-zeros network address, and one all-ones broadcast network address.

The table below shows an example of the four IP addresses that are created when a 30-bit (otherwise known as 255.255.255.252 or /30) subnet mask is applied to the IP address 192.168.100.4. The bits that are used to specify the host IP addresses in bold.

Point-to-point links only have two endpoints (hosts) and do not require broadcast support because any packet that is transmitted by one host is always received by the other host. Therefore the all-ones broadcast IP address is not required for a point-to-point interface.

The simplest way to explain RFC 3021 is to say that the use of a 31-bit prefix (created by applying a 31-bit subnet mask to an IP address) allows the all-zeros and all-ones IP addresses to be assigned as host addresses on point-to-point networks. Prior to RFC 3021 the longest prefix in common use on point-to-point links was 30-bits, which meant that the all-zeros and all-ones IP addresses were wasted.

The table below shows an example of the two IP addresses that are created when a 31-bit (otherwise known as 255.255.255.254 or /31) subnet mask is applied to the IP address 192.168.100.4. The bit that is used to specify the host IP addresses in bold

The complete text for RFC 3021 is available at http:/​/​www.ietf.org/​rfc/​rfc3021.txt .

You must have classless IP addressing configured on your networking device before you configure an IP address with a 31-bit prefix on a point-to-point interface. Classless IP addressing is enabled by default in many versions of Cisco IOS software. If you are not certain that your networking device has IP classless addressing configured, enter the ip classless command in global configuration mode to enable it.

3.     ip classless

Specifies a point-to-point WAN interface and enters interface configuration mode.

Enables the interface.

Configures the 31bit prefix IP address on the point-to-point WAN interface.

Exits the current configuration mode and returns to privileged EXEC mode.

The following example configures an IP address on three interfaces:

The following example configures secondary IP addresses on three interfaces:

The following example configures the unnumbered IP feature on three interfaces:

The following example configures 31-bit prefixes on two interfaces:

The following example enables subnet zero:

If your network has two or more routers and you have not already configured a routing protocol, consult the Cisco IOS IP Routing Protocols Configuration Guide , Release 12.4T, for information on configuring routing protocols.

Related Documents

Technical assistance.

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Contents Configuring IPv4 Addresses Finding Feature Information Information About IP Addresses Binary Numbering IP Address Structure IP Address Classes IP Network Subnetting IP Network Address Assignments Classless Inter-Domain Routing Prefixes How to Configure IP Addresses Establishing IP Connectivity to a Network by Assigning an IP Address to an Interface Troubleshooting Tips Increasing the Number of IP Hosts that Are Supported on a Network by Using Secondary IP Addresses Troubleshooting Tips What to Do Next Maximizing the Number of Available IP Subnets by Allowing the Use of IP Subnet Zero Troubleshooting Tips Specifying the Format of Network Masks Specifying the Format in Which Netmasks Appear for the Current Session Specifying the Format in Which Netmasks Appear for an Individual Line Using IP Unnumbered Interfaces on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required IP Unnumbered Feature Troubleshooting Tips Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required RFC 3021 Troubleshooting Tips Configuration Examples for IP Addresses Example Establishing IP Connectivity to a Network by Assigning an IP Address to an Interface Example Increasing the Number of IP Hosts that are Supported on a Network by Using Secondary IP Addresses Example Using IP Unnumbered Interfaces on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required Example Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required Example Maximizing the Number of Available IP Subnets by Allowing the Use of IP Subnet Zero Where to Go Next Additional References Feature Information for IP Addresses Configuring IPv4 Addresses This chapter contains information about, and instructions for configuring IPv4 addresses on interfaces that are part of a networking device. Note All further references to IPv4 addresses in this document use only IP in the text, not IPv4. Finding Feature Information Information About IP Addresses How to Configure IP Addresses Configuration Examples for IP Addresses Where to Go Next Additional References Feature Information for IP Addresses Finding Feature Information Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn . An account on Cisco.com is not required. Information About IP Addresses Binary Numbering IP Address Structure IP Address Classes IP Network Subnetting IP Network Address Assignments Classless Inter-Domain Routing Prefixes Binary Numbering IP addresses are 32 bits long. The 32 bits are divided into four octets (8-bits). A basic understanding of binary numbering is very helpful if you are going to manage IP addresses in a network because changes in the values of the 32 bits indicate either a different IP network address or IP host address. A value in binary is represented by the number (0 or 1) in each position multiplied by the number 2 to the power of the position of the number in sequence, starting with 0 and increasing to 7, working right to left. The figure below is an example of an 8-digit binary number. Figure 1. Example of an 8-digit Binary Number The figure below provides binary to decimal number conversion for 0 through 134. Figure 2. Binary to Decimal Number Conversion for 0 to 134 The figure below provides binary to decimal number conversion for 135 through 255. Figure 3. Binary to Decimal Number Conversion for 135 to 255 IP Address Structure An IP host address identifies a device to which IP packets can be sent. An IP network address identifies a specific network segment to which one or more hosts can be connected. The following are characteristics of IP addresses: IP addresses are 32 bits long IP addresses are divided into four sections of one byte (octet) each IP addresses are typically written in a format known as dotted decimal The table below shows some examples of IP addresses. Table 1 Examples of IP Addresses IP Addresses in Dotted Decimal IP Addresses in Binary 10.34.216.75 00001010.00100010.11011000.01001011 172.16.89.34 10101100.00010000.01011001.00100010 192.168.100.4 11000000.10101000.01100100.00000100 Note The IP addresses in the table above are from RFC 1918, Address Allocation for Private Internets . These IP addresses are not routable on the Internet. They are intended for use in private networks. For more information on RFC1918, see http:/​/​www.ietf.org/​rfc/​rfc1918.txt . IP addresses are further subdivided into two sections known as network and host. The division is accomplished by arbitrarily ranges of IP addresses to classes. For more information see RFC 791 Internet Protocol at http:/​/​www.ietf.org/​rfc/​rfc0791.txt . IP Address Classes In order to provide some structure to the way IP addresses are assigned, IP addresses are grouped into classes. Each class has a range of IP addresses. The range of IP addresses in each class is determined by the number of bits allocated to the network section of the 32-bit IP address. The number of bits allocated to the network section is represented by a mask written in dotted decimal or with the abbreviation / n where n = the numbers of bits in the mask. The table below lists ranges of IP addresses by class and the masks associated with each class. The digits in bold indicate the network section of the IP address for each class. The remaining digits are available for host IP addresses. For example, IP address 10.90.45.1 with a mask of 255.0.0.0 is broken down into a network IP address of 10.0.0.0 and a host IP address of 0.90.45.1. Table 2 IP Address Ranges by Class with Masks Class Range A (range/mask in dotted decimal) 0 .0.0.0 to 127.0.0.0/8 (255.0.0.0) A (range in binary) 00000000 .00000000.00000000.00000000 to 01111111 .00000000.00000000.00000000 A (mask in binary) 11111111.00000000.00000000.00000000/8 B (range/mask in dotted decimal) 128 .0.0.0 to 191 . 255 .0.0/16 (255.255.0.0) B (range in binary) 10000000 . 00000000 .00000000.00000000 to 10111111 . 11111111 .00000000.00000000 B (mask in binary) 11111111 . 11111111 .00000000.00000000/16 C (range/mask in dotted decimal) 192 . 0 . 0 .0 to 223 . 255 . 255 .0/24 (255.255.255.0) C (range in binary) 11000000 . 00000000 . 00000000 .00000000 to 11011111 . 11111111 . 11111111 .00000000 C (mask in binary) 11111111.11111111.11111111.0000000/24 D 1 (range/mask in dotted decimal) 224 . 0 . 0 . 0 to 239 . 255 . 255 . 255 /32 (255.255.255.255) D (range in binary) 11100000 . 00000000 . 00000000 . 00000000 to 11101111 . 11111111 . 11111111 . 11111111 D (mask in binary) 11111111.11111111.11111111.11111111/32 E 2 (range/mask in dotted decimal) 240 . 0 . 0 . 0 to 255 . 255 . 255 . 255 /32 (255.255.255.255) E (range in binary) 11110000 . 00000000 . 00000000 . 00000000 to 11111111 . 11111111 . 11111111 . 11111111 E (mask in binary) 11111111.11111111.11111111.11111111/32 1 Class D IP addresses are reserved for multicast applications. 2 Class E IP addresses are reserved for broadcast traffic. Note Some IP addresses in these ranges are reserved for special uses. For more information refer to RFC 3330, Special-Use IP Addresses , at http:/​/​www.ietf.org/​rfc/​rfc3330.txt . When a digit that falls within the network mask changes from 1 to 0 or 0 to 1 the network address is changed. For example, if you change 10101100.00010000.01011001.00100010/16 to 10101100.00110000.01011001.00100010/16 you have changed the network address from 172.16.89.34/16 to 172.48.89.34/16. When a digit that falls outside the network mask changes from 1 to 0 or 0 to 1 the host address is changed. For example, if you change 10101100.00010000.01011001.00100010/16 to 10101100.00010000.01011001.00100011/16 you have changed the host address from 172.16.89.34/16 to 172.16.89.35/16. Each class of IP address supports a specific range of IP network addresses and IP host addresses. The range of IP network addresses available for each class is determined with the formula 2 to the power of the number of available bits. In the case of class A addresses, the value of the first bit in the 1st octet (as shown in the table above) is fixed at 0. This leaves 7 bits for creating additional network addresses. Therefore there are 128 IP network addresses available for class A (27 = 128). The number of IP host addresses available for an IP address class is determined by the formula 2 to the power of the number of available bits minus 2. There are 24 bits available in a class A addresses for IP host addresses. Therefore there are 16,777,214 IP hosts addresses available for class A ((224) - 2 = 16,777,214)). Note The 2 is subtracted because there are 2 IP addresses that cannot be used for a host. The all 0’s host address cannot be used because it is the same as the network address. For example, 10.0.0.0 cannot be both a IP network address and an IP host address. The all 1’s address is a broadcast address that is used to reach all hosts on the network. For example, an IP datagram addressed to 10.255.255.255 will be accepted by every host on network 10.0.0.0. The table below shows the network and host addresses available for each class of IP address. Table 3 Network and Host Addresses Available for Each Class of IP Address Class Network Addresses Host Addresses A 128 16,777,214 B 16,384 3 65534 C 2,097,152 4 254 3 Only 14 bits are available for class B IP network addresses because the first 2 bits are fixed at 10 as shown in Table 2 . 4 Only 21 bits are available for class C IP network addresses because the first 3 bits are fixed at 110 as shown in Table 2 . IP Network Subnetting The arbitrary subdivision of network and host bits in IP address classes resulted in an inefficient allocation of IP space. For example, if your network has 16 separate physical segments you will need 16 IP network addresses. If you use 16 class B IP network addresses, you would be able to support 65,534 hosts on each of the physical segments. Your total number of supported host IP addresses is 1,048,544 (16 * 65,534 = 1,048,544). Very few network technologies can scale to having 65,534 hosts on a single network segment. Very few companies need 1,048,544 IP host addresses. This problem required the development of a new strategy that permitted the subdivision of IP network addresses into smaller groupings of IP subnetwork addresses. This strategy is known as subnetting. If your network has 16 separate physical segments you will need 16 IP subnetwork addresses. This can be accomplished with one class B IP address. For example, start with the class B IP address of 172.16.0.0 you can reserve 4 bits from the third octet as subnet bits. This gives you 16 subnet IP addresses 24 = 16. The table below shows the IP subnets for 172.16.0.0/20. Table 4 Examples of IP Subnet Addresses using 172.16.0.0/20 Number IP Subnet Addresses in Dotted Decimal IP Subnet Addresses in Binary 0 5 172.16.0.0 10101100.00010000.00000000.00000000 1 172.16.16.0 10101100.00010000.00010000.00000000 2 172.16.32.0 10101100.00010000.00100000.00000000 3 172.16.48.0 10101100.00010000.00110000.00000000 4 172.16.64.0 10101100.00010000.01000000.00000000 5 172.16.80.0 10101100.00010000.01010000.00000000 6 172.16.96.0 10101100.00010000.01100000.00000000 7 172.16.112.0 10101100.00010000.01110000.00000000 8 172.16.128.0 10101100.00010000.10000000.00000000 9 172.16.144.0 10101100.00010000.10010000.00000000 10 172.16.160.0 10101100.00010000.10100000.00000000 11 172.16.176.0 10101100.00010000.10110000.00000000 12 172.16.192.0 10101100.00010000.11000000.00000000 13 172.16.208.0 10101100.00010000.11010000.00000000 14 172.16.224.0 10101100.00010000.11100000.00000000 15 172.16.240.0 10101100.00010000.11110000.00000000 5 The first subnet that has all of the subnet bits set to 0 is referred to as subnet 0 . It is indistinguishable from the network address and must be used carefully. When a digit that falls within the subnetwork (subnet) mask changes from 1 to 0 or 0 to 1 the subnetwork address is changed. For example, if you change 10101100.00010000.01011001.00100010/20 to 10101100.00010000.01111001.00100010/20 you have changed the network address from 172.16.89.34/20 to 172.16.121.34/20. When a digit that falls outside the subnet mask changes from 1 to 0 or 0 to 1 the host address is changed. For example, if you change 10101100.00010000.01011001.00100010/20 to 10101100.00010000.01011001.00100011/20 you have changed the host address from 172.16.89.34/20 to 172.16.89.35/20. Timesaver To avoid having to do manual IP network, subnetwork, and host calculations, use one of the free IP subnet calculators available on the Internet. Some people get confused about the terms network address and subnet or subnetwork addresses and when to use them. In the most general sense the term network address means “the IP address that routers use to route traffic to a specific network segment so that the intended destination IP host on that segment can receive it”. Therefore the term network address can apply to both non-subnetted and subnetted IP network addresses. When you are troubleshooting problems with forwarding traffic from a router to a specific IP network address that is actually a subnetted network address, it can help to be more specific by referring to the destination network address as a subnet network address because some routing protocols handle advertising subnet network routes differently from network routes. For example, the default behavior for RIP v2 is to automatically summarize the subnet network addresses that it is connected to their non-subnetted network addresses (172.16.32.0/24 is advertised by RIP v2 as 172.16.0.0/16) when sending routing updates to other routers. Therefore the other routers might have knowledge of the IP network addresses in the network, but not the subnetted network addresses of the IP network addresses. Tip The term IP address space is sometimes used to refer to a range of IP addresses. For example, “We have to allocate a new IP network address to our network because we have used all of the available IP addresses in the current IP address space”. IP Network Address Assignments Routers keep track of IP network addresses to understand the network IP topology (layer 3 of the OSI reference model) of the network to ensure that IP traffic can be routed properly. In order for the routers to understand the network layer (IP) topology, every individual physical network segment that is separated from any other physical network segment by a router must have a unique IP network address. The figure below shows an example of a simple network with correctly configured IP network addresses. The routing table in R1 looks like the table below. Table 5 Routing Table for a Correctly Configured Network Interface Ethernet 0 Interface Ethernet 1 172.31.32.0/24 (Connected) 172.31.16.0/24 (Connected) Figure 4. Correctly Configured Network The figure below shows an example of a simple network with incorrectly configured IP network addresses. The routing table in R1 looks like the table below. If the PC with IP address 172.31.32.3 attempts to send IP traffic to the PC with IP address 172.31.32.54, router R1 cannot determine which interface that the PC with IP address 172.31.32.54 is connected to. Table 6 Routing Table in Router R1 for an Incorrectly Configured Network (Example 1) Ethernet 0 Ethernet 1 172.31.32.0/24 (Connected) 172.31.32.0/24 (Connected) Figure 5. Incorrectly Configured Network (Example 1) To help prevent mistakes as shown in the figure above, Cisco IOS-based networking devices will not allow you to configure the same IP network address on two or more interfaces in the router when IP routing is enabled. The only way to prevent the mistake shown in the figure below, where 172.16.31.0/24 is used in R2 and R3, is to have very accurate network documentation that shows where you have assigned IP network addresses. Table 7 Routing Table in Router R1 for an Incorrectly Configured Network (Example 2) Ethernet 0 Serial 0 Serial 1 172.16.32.0/24 (Connected) 192.168.100.4/29 (Connected) 172.16.31.0/24 RIP 192.168.100.8/29 (Connected) 172.16.31.0/24 RIP Figure 6. Incorrectly Configured Network (Example 2) For a more thorough explanation of IP routing, see the "Related Documents" section for a list of documents related to IP routing. Classless Inter-Domain Routing Due to the continuing increase in internet use and the limitations on how IP addresses can be assigned using the class structure shown in the table above, a more flexible method for allocating IP addresses was required. The new method is documented in RFC 1519 Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy . CIDR allows network administrators to apply arbitrary masks to IP addresses to create an IP addressing plan that meets the requirements of the networks that they administrate. For more information on CIDR, refer to RFC 1519 at http:/​/​www.ietf.org/​rfc/​rfc1519.txt . Prefixes The term prefix is often used to refer to the number of bits of an IP network address that are of importance for building routing tables. If you are using only classful (strict adherence to A, B, and C network address boundaries) IP addresses, the prefixes are the same as the masks for the classes of addresses. For example, using classful IP addressing, a class C IP network address such as 192.168.10.0 uses a 24-bit mask (/24 or 255.255.255.0) and can also be said to have a 24-bit prefix. If you are using CIDR, the prefixes are arbitrarily assigned to IP network addresses based on how you want to populate the routing tables in your network. For example, a group of class C IP addresses such as 192.168.10.0, 192.168.11.0, 192.168.12.0, 192.168.13.0 can be advertised as a single route to 192.168.0.0 with a 16-bit prefix (192.168.0.0/16). This results in a 4:1 reduction in the number of routes that the routers in your network need to manage. How to Configure IP Addresses Establishing IP Connectivity to a Network by Assigning an IP Address to an Interface Increasing the Number of IP Hosts that Are Supported on a Network by Using Secondary IP Addresses Maximizing the Number of Available IP Subnets by Allowing the Use of IP Subnet Zero Specifying the Format of Network Masks Using IP Unnumbered Interfaces on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required Establishing IP Connectivity to a Network by Assigning an IP Address to an Interface Perform this task to configure an IP address on an interface. SUMMARY STEPS 1.     enable 2.     configure terminal 3.     interface type number 4.     no shutdown 5.     ip address ip-address mask 6.     end DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 configure terminal Example: Router# configure terminal   Enters global configuration mode.   Step 3 interface type number Example: Router(config)# interface fastethernet 0/0   Specifies an interface and enters interface configuration mode.   Step 4 no shutdown Example: Router(config-if)# no shutdown   Enables the interface.   Step 5 ip address ip-address mask Example: Router(config-if)# ip address 172.16.16.1 255.255.240.0   Configures the IP address on the interface.   Step 6 end Example: Router(config-if)# end   Exits the current configuration mode and returns to privileged EXEC mode.   Troubleshooting Tips The following commands can help troubleshoot IP addressing: show ip interface --Displays the IP parameters for the interface. show ip route connected --Displays the IP networks the networking device is connected to. Increasing the Number of IP Hosts that Are Supported on a Network by Using Secondary IP Addresses If you have a situation in which you need to connect more IP hosts to a network segment and you have used all of the available IP host addresses for the subnet to which you have assigned the segment, you can avoid having to readdress all of the hosts with a different subnet by adding a second IP network address to the network segment. Perform this task to configure a secondary IP address on an interface. SUMMARY STEPS 1.     enable 2.     configure terminal 3.     interface type number 4.     no shutdown 5.     ip address ip-address mask 6.     ip address ip-address mask secondary 7.     end DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 configure terminal Example: Router# configure terminal   Enters global configuration mode.   Step 3 interface type number Example: Router(config)# interface fastethernet 0/0   Specifies an interface and enters interface configuration mode.   Step 4 no shutdown Example: Router(config-if)# no shutdown   Enables the interface.   Step 5 ip address ip-address mask Example: Router(config-if)# ip address 172.16.16.1 255.255.240.0   Configures the IP address on the interface.   Step 6 ip address ip-address mask secondary Example: Router(config-if)# ip address 172.16.32.1 255.255.240.0 secondary   Configures the secondary IP address on the interface.   Step 7 end Example: Router(config-if)# end   Exits the current configuration mode and returns to privileged EXEC mode.   Troubleshooting Tips The following commands can help troubleshoot IP addressing: show ip interface --Displays the IP parameters for the interface. show ip route connected --Displays the IP networks the networking device is connected to. What to Do Next If your network has two or more routers and you have already configured a routing protocol, make certain that the other routers can reach the new IP network that you assigned. You might need to modify the configuration for the routing protocol on the router so that it advertises the new network. Consult the Cisco IOS IP Routing: Protocol-Independent Configuration Guide for information on configuring routing protocols. Maximizing the Number of Available IP Subnets by Allowing the Use of IP Subnet Zero If you using subnetting in your network and you are running out of network addresses, you can configure your networking device to allow the configuration of subnet zero. This adds one more usable network address for every subnet in your IP addressing scheme. The table above shows the IP subnets (including subnet 0) for 172.16.0.0/20. Perform this task to enable the use of IP subnet zero on your networking device. SUMMARY STEPS 1.     enable 2.     configure terminal 3.     ip subnet-zero 4.     interface type number 5.     no shutdown 6.     ip address ip-address mask 7.     end DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 configure terminal Example: Router# configure terminal   Enters global configuration mode.   Step 3 ip subnet-zero Example: Router(config)# ip subnet-zero   Enables the use of IP subnet zero.   Step 4 interface type number Example: Router(config)# interface fastethernet 0/0   Specifies an interface and enters interface configuration mode.   Step 5 no shutdown Example: Router(config-if)# no shutdown   Enables the interface.   Step 6 ip address ip-address mask Example: Router(config-if)# ip address 172.16.0.1 255.255.240.0   Configures the subnet zero IP address on the interface.   Step 7 end Example: Router(config-if)# end   Exits the current configuration mode and returns to privileged EXEC mode.   Troubleshooting Tips The following commands can help troubleshoot IP addressing: show ip interface --Displays the IP parameters for the interface. show ip route connected --Displays the IP networks the networking device is connected to. Specifying the Format of Network Masks By default, show commands display an IP address and then its netmask in dotted decimal notation. For example, a subnet would be displayed as 131.108.11.55 255.255.255.0. You might find it more convenient to display the network mask in hexadecimal format or bit count format instead. The hexadecimal format is commonly used on UNIX systems. The previous example would be displayed as 131.108.11.55 0XFFFFFF00. The bit count format for displaying network masks is to append a slash (/) and the total number of bits in the netmask to the address itself. The previous example would be displayed as 131.108.11.55/24. Specifying the Format in Which Netmasks Appear for the Current Session Specifying the Format in Which Netmasks Appear for an Individual Line Specifying the Format in Which Netmasks Appear for the Current Session Perform this task to specify the format in which netmasks appear for the current session. SUMMARY STEPS 1.     enable 2.     term ip netmask-format { bitcount | decimal | hexadecimal } DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 term ip netmask-format { bitcount | decimal | hexadecimal } Example: Router# term ip netmask-format hexadecimal   Specifies the format the router uses to display network masks.   Specifying the Format in Which Netmasks Appear for an Individual Line Perform this task to specify the format in which netmasks appear for an individual line. SUMMARY STEPS 1.     enable 2.     configure terminal 3.     line vty first last 4.     term ip netmask-format { bitcount | decimal | hexadecimal } 5.     end DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 configure terminal Example: Router# configure terminal   Enters global configuration mode.   Step 3 line vty first last Example: Router(config)# line vty 0 4   Enters line configuration mode for the range of lines specified by the first and last arguments.   Step 4 term ip netmask-format { bitcount | decimal | hexadecimal } Example: Router(config-line)# ip netmask-format hexadecimal   Specifies the format the router uses to display the network mask for an individual line.   Step 5 end Example: Router(config-if)# end   Exits the current configuration mode and returns to privileged EXEC mode.   Using IP Unnumbered Interfaces on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required If you have a limited number of IP network or subnet addresses and you have point-to-point WANs in your network, you can use the IP Unnumbered Interfaces feature to enable IP connectivity on the point-to-point WAN interfaces without actually assigning an IP address to them. Perform this task to configure the IP Unnumbered Interfaces feature on a point-to-point WAN interface. IP Unnumbered Feature Troubleshooting Tips IP Unnumbered Feature The IP Unnumbered Interfaces feature enables IP processing on a point-to-point WAN interface without assigning it an explicit IP address. The IP unnumbered point-to-point WAN interface uses the IP address of another interface to enable IP connectivity, which conserves network addresses. Note The following restrictions apply to the IP Unnumbered Interfaces feature: The IP Unnumbered Interfaces feature is only supported on point-to-point (non-multiaccess) WAN interfaces You cannot netboot a Cisco IOS image over an interface that is using the IP Unnumbered Interfaces feature SUMMARY STEPS 1.     enable 2.     configure terminal 3.     interface type number 4.     no shutdown 5.     ip address ip-address mask 6.     interface type number 7.     no shutdown 8.     ip unnumbered type number 9.     end DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 configure terminal Example: Router# configure terminal   Enters global configuration mode.   Step 3 interface type number Example: Router(config)# interface fastethernet 0/0   Specifies an interface and enters interface configuration mode.   Step 4 no shutdown Example: Router(config-if)# no shutdown   Enables the interface.   Step 5 ip address ip-address mask Example: Router(config-if)# ip address 172.16.16.1 255.255.240.0   Configures the IP address on the interface.   Step 6 interface type number Example: Router(config-if)# interface serial 0/0   Specifies a point-to-point WAN interface and enters interface configuration mode.   Step 7 no shutdown Example: Router(config-if)# no shutdown   Enables the point-to-point WAN interface.   Step 8 ip unnumbered type number Example: Router(config-if)# ip unnumbered fastethernet 0/0   Enables the IP unnumbered feature on the point-to-point WAN interface. In this example the point-to-point WAN interface uses IP address 172.16.16.1 from Fast Ethernet 0/0.   Step 9 end Example: Router(config-if)# end   Exits the current configuration mode and returns to privileged EXEC mode.   Troubleshooting Tips The following commands can help troubleshoot IP addressing: show ip interface --Displays the IP parameters for the interface. show ip route connected --Displays the IP networks the networking device is connected to. Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required You can reduce the number of IP subnets used by networking devices to establish IP connectivity to point-to-point WANs that they are connected to by using IP Addresses with 31-bit Prefixes as defined in RFC 3021. Perform this task to configure an IP address with a 31-bit prefix on a point-to-point WAN interface. RFC 3021 Troubleshooting Tips RFC 3021 Prior to RFC 3021, Using 31-bit Prefixes on IPv4 Point-to-Point Links , many network administrators assigned IP address with a 30-bit subnet mask (255.255.255.252) to point-to-point interfaces to conserve IP address space. Although this practice does conserve IP address space compared to assigning IP addresses with shorter subnet masks such as 255.255.255.240, IP addresses with a 30-bit subnet mask still require four addresses per link: two host addresses (one for each host interface on the link), one all-zeros network address, and one all-ones broadcast network address. The table below shows an example of the four IP addresses that are created when a 30-bit (otherwise known as 255.255.255.252 or /30) subnet mask is applied to the IP address 192.168.100.4. The bits that are used to specify the host IP addresses in bold. Table 8 Four IP Addresses Created When a 30-Bit Subnet Mask (/30) Is Used Address Description Binary 192.168.100.4/30 All-zeros IP address 11000000.10101000.01100100.000001 00 192.168.100.5/30 First host addresses 11000000.10101000.01100100.000001 01 192.168.100.6/30 Second host address 11000000.10101000.01100100.000001 10 192.168.100.7/30 All-ones broadcast address 11000000.10101000.01100100.000001 11 Point-to-point links only have two endpoints (hosts) and do not require broadcast support because any packet that is transmitted by one host is always received by the other host. Therefore the all-ones broadcast IP address is not required for a point-to-point interface. The simplest way to explain RFC 3021 is to say that the use of a 31-bit prefix (created by applying a 31-bit subnet mask to an IP address) allows the all-zeros and all-ones IP addresses to be assigned as host addresses on point-to-point networks. Prior to RFC 3021 the longest prefix in common use on point-to-point links was 30-bits, which meant that the all-zeros and all-ones IP addresses were wasted. The table below shows an example of the two IP addresses that are created when a 31-bit (otherwise known as 255.255.255.254 or /31) subnet mask is applied to the IP address 192.168.100.4. The bit that is used to specify the host IP addresses in bold Table 9 Two IP Addresses Created When a 31-Bit Subnet Mask (/31) Is Used Address Description Binary 192.168.100.4/31 First host address 11000000.10101000.01100100.0000010 0 192.168.100.5/31 Second host address 11000000.10101000.01100100.0000010 1 The complete text for RFC 3021 is available at http:/​/​www.ietf.org/​rfc/​rfc3021.txt . Before You Begin You must have classless IP addressing configured on your networking device before you configure an IP address with a 31-bit prefix on a point-to-point interface. Classless IP addressing is enabled by default in many versions of Cisco IOS software. If you are not certain that your networking device has IP classless addressing configured, enter the ip classless command in global configuration mode to enable it. Note This task can only be performed on point-to-point (nonmultiaccess) WAN interfaces. SUMMARY STEPS 1.     enable 2.     configure terminal 3.     ip classless 4.     interface type number 5.     no shutdown 6.     ip address ip-address mask 7.     end DETAILED STEPS   Command or Action Purpose Step 1 enable Example: Router> enable   Enables privileged EXEC mode. Enter your password if prompted.   Step 2 configure terminal Example: Router# configure terminal   Enters global configuration mode.   Step 3 ip classless Example: Router(config)# ip classless   (Optional) Enables IP classless (CIDR). Note    This command is enabled by default in many versions of Cisco IOS. If you are not certain if it is enabled by default in the version of Cisco IOS that your networking device is running, enter the ip classless command as shown. When you are done with this task view the configuration. If the ip classless command does not appear in your configuration, it is enabled by default.   Step 4 interface type number Example: Router(config)# interface serial 0/0   Specifies a point-to-point WAN interface and enters interface configuration mode.   Step 5 no shutdown Example: Router(config-if)# no shutdown   Enables the interface.   Step 6 ip address ip-address mask Example: Router(config-if)# ip address 192.168.100.4 255.255.255.254   Configures the 31bit prefix IP address on the point-to-point WAN interface.   Step 7 end Example: Router(config-if)# end   Exits the current configuration mode and returns to privileged EXEC mode.   Troubleshooting Tips The following commands can help troubleshoot IP addressing: show ip interface --Displays the IP parameters for the interface. show ip route connected --Displays the IP networks the networking device is connected to. Configuration Examples for IP Addresses Example Establishing IP Connectivity to a Network by Assigning an IP Address to an Interface Example Increasing the Number of IP Hosts that are Supported on a Network by Using Secondary IP Addresses Example Using IP Unnumbered Interfaces on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required Example Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required Example Maximizing the Number of Available IP Subnets by Allowing the Use of IP Subnet Zero Example Establishing IP Connectivity to a Network by Assigning an IP Address to an Interface The following example configures an IP address on three interfaces: ! interface FastEthernet0/0 no shutdown ip address 172.16.16.1 255.255.240.0 ! interface FastEthernet0/1 no shutdown ip address 172.16.32.1 255.255.240.0 ! interface FastEthernet0/2 no shutdown ip address 172.16.48.1 255.255.240.0 ! Example Increasing the Number of IP Hosts that are Supported on a Network by Using Secondary IP Addresses The following example configures secondary IP addresses on three interfaces: ! interface FastEthernet0/0 no shutdown ip address 172.16.16.1 255.255.240.0 ip address 172.16.32.1 255.255.240.0 secondary ! ! interface FastEthernet0/1 no shutdown ip address 172.17.16.1 255.255.240.0 ip address 172.17.32.1 255.255.240.0 secondary ! ! interface FastEthernet0/2 no shutdown ip address 172.18.16.1 255.255.240.0 ip address 172.18.32.1 255.255.240.0 secondary ! Example Using IP Unnumbered Interfaces on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required The following example configures the unnumbered IP feature on three interfaces: ! interface FastEthernet0/0 no shutdown ip address 172.16.16.1 255.255.240.0 ! interface serial0/0 no shutdown ip unnumbered fastethernet0/0 ! interface serial0/1 no shutdown ip unnumbered fastethernet0/0 ! interface serial0/2 no shutdown ip unnumbered fastethernet0/0 ! Example Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces to Limit Number of IP Addresses Required The following example configures 31-bit prefixes on two interfaces: ! ip classless ! interface serial0/0 no shutdown ip address 192.168.100.2 255.255.255.254 ! ! interface serial0/1 no shutdown ip address 192.168.100.4 255.255.255.254 Example Maximizing the Number of Available IP Subnets by Allowing the Use of IP Subnet Zero The following example enables subnet zero: ! interface FastEthernet0/0 no shutdown ip address 172.16.16.1 255.255.240.0 ! ip subnet-zero ! Where to Go Next If your network has two or more routers and you have not already configured a routing protocol, consult the Cisco IOS IP Routing Protocols Configuration Guide , Release 12.4T, for information on configuring routing protocols. Additional References Related Documents Related Topic Document Title Cisco IOS commands Cisco IOS Master Commands List, All Releases IP addressing commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples Cisco IOS IP Addressing Services Command Reference Fundamental principles of IP addressing and IP routing IP Routing Primer ISBN 1578701082 Standards Standard Title No new or modified standards are supported, and support for existing standards has not been modified -- MIBs MIB MIBs Link No new or modified MIBs are supported, and support for existing MIBs has not been modified To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL: http:/​/​www.cisco.com/​go/​mibs RFCs RFC 6 Title RFC 791 Internet Protocol http:/​/​www.ietf.org/​rfc/​rfc0791.txt RFC 1338 Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy http:/​/​www.ietf.org/​rfc/​rfc1519.txt RFC 1466 Guidelines for Management of IP Address Space http:/​/​www.ietf.org/​rfc/​rfc1466.txt RFC 1716 Towards Requirements for IP Routers http:/​/​www.ietf.org/​rfc/​rfc1716.txt RFC 1918 Address Allocation for Private Internets http:/​/​www.ietf.org/​rfc/​rfc1918.txt RFC 3330 Special-Use IP Addresses http:/​/​www.ietf.org/​rfc/​rfc3330.txt 6 These references are only a sample of the many RFCs available on subjects related to IP addressing and IP routing. Refer to the IETF RFC site at http://www.ietf.org/rfc.html for a full list of RFCs. Technical Assistance Description Link The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. http:/​/​www.cisco.com/​cisco/​web/​support/​index.htmll Feature Information for IP Addresses The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn . An account on Cisco.com is not required. Table 10 Feature Information for IP Addresses Feature Name Releases Feature Information Classless Inter-Domain Routing 10.0 CIDR is a new way of looking at IP addresses that eliminates the concept of classes (class A, class B, and so on). For example, network 192.213.0.0, which is an illegal class C network number, is a legal supernet when it is represented in CIDR notation as 192.213.0.0/16. The /16 indicates that the subnet mask consists of 16 bits (counting from the left). Therefore, 192.213.0.0/16 is similar to 192.213.0.0 255.255.0.0. The following command was introduced or modified: ip classless . IP Subnet Zero 10.0 In order to conserve IP address space IP Subnet Zero allows the use of the all-zeros subnet as an IP address on an interface, such as configuring 172.16.0.1/24 on Fast Ethernet 0/0. The following command was introduced or modified: ip subnet-zero . IP Unnumbered Interfaces 10.0 In order to conserve IP address space, IP unnumbered interfaces use the IP address of another interface to enable IP connectivity. The following command was introduced or modified: ip unnumbered . Using 31-bit Prefixes on IP Point-to-Point Links 12.0(14)S 12.2(4)T In order to conserve IP address space on the Internet, a 31-bit prefix length allows the use of only two IP addresses on a point-to-point link. Previously, customers had to use four IP addresses or unnumbered interfaces for point-to-point links.

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Change TCP/IP settings

TCP/IP defines how your PC communicates with other PCs.

To make it easier to manage TCP/IP settings, we recommend using automated Dynamic Host Configuration Protocol (DHCP). DHCP automatically assigns Internet Protocol (IP) addresses to the computers on your network if your network supports it. If you use DHCP, then you don't have to change your TCP/IP settings if you move your PC to another location, and DHCP doesn't require you to manually configure TCP/IP settings, such as Domain Name System (DNS) and Windows Internet Name Service (WINS).

To enable DHCP or change other TCP/IP settings

Select  Start , then type settings . Select  Settings  >  Network & internet .

Do one of the following:

For a Wi-Fi network, select  Wi-Fi  >  Manage known networks . Choose the network for which you want to change the settings.

For an Ethernet network, select  Ethernet , then select the Ethernet network you’re connected to.

Next to  IP assignment , select  Edit .

Under  Edit network IP settings or Edit IP settings , select  Automatic (DHCP)  or  Manual .

To specify IPv4 settings manually

Under  Edit network IP settings or Edit IP settings , choose  Manual , then turn on  IPv4 .

To specify an IP address, in the  IP address, Subnet mask , and  Gateway  boxes, type the IP address settings.

To specify a DNS server address, in the  Preferred DNS  and  Alternate DNS  boxes, type the addresses of the primary and secondary DNS servers.

To specify if you want to use an encrypted (DNS over HTTPS) or unencrypted connection to the DNS server or servers you specify, for DNS over HTTPS , choose the setting you want:

Off : All DNS queries will be sent to the DNS server unencrypted in plaintext over HTTP.

On (automatic template) : DNS queries will be encrypted and sent to the DNS server over HTTPS. DNS queries will use the default settings for the automatic template or try to discover them automatically.

On (manual template) : DNS queries will be encrypted and sent to the DNS server over HTTPS. They’ll use the settings you enter in the DNS over HTTPS template box.

If you use DNS over HTTPS (automatic or manual template), turn Fallback to plaintext on or off:

When it’s turned on, a DNS query will be sent unencrypted if it can’t be sent over HTTPS.

When it’s turned off, a DNS query won’t be sent if it can’t be sent over HTTPS.

To specify IPv6 settings manually

Under  Edit network IP settings or Edit IP settings , choose  Manual , then turn on  IPv6 .

To specify an IP address, in the  IP address , Subnet prefix length , and  Gateway  boxes, type the IP address settings.

When you select  Automatic (DHCP) , the IP address settings and DNS server address setting are set automatically by your router or other access point (recommended).

When you select  Manual , you can manually set your IP address settings and DNS server address.

When you’re done, select  Save .

Note:  To install IPv4, run Command Prompt as an administrator, type netsh interface ipv4 install , and then press Enter .

Select Start   , then select Settings   > Network & Internet   .

For a Wi-Fi network, select Wi-Fi   > Manage known networks . Choose the network you want to change the settings for, then select Properties.

For an Ethernet network, select Ethernet , then select the Ethernet network you’re connected to.

Under IP assignment , select Edit .

Under Edit IP settings , select Automatic (DHCP) or Manual .

Under Edit IP settings , choose Manual , then turn on IPv4 .

To specify an IP address, in the IP address, Subnet prefix length , and Gateway boxes, type the IP address settings.

To specify a DNS server address, in the Preferred DNS and Alternate DNS boxes, type the addresses of the primary and secondary DNS servers.

Under Edit IP settings , choose Manual , then turn on IPv6 .

When you select Automatic (DHCP) , the IP address settings and DNS server address setting are set automatically by your router or other access point (recommended).

When you select Manual , you can manually set your IP address settings and DNS server address.

When you’re done, select Save .

In Windows 8.1, select the Start button, start typing View network connections , and then select View network connections in the list.

In Windows 7, open Network Connections by selecting the Start button, and then selecting Control Panel . In the search box, type adapter , and then, under Network and Sharing Center , select View network connections .

Right-click the connection that you want to change, and then select Properties . If you're prompted for an administrator password or confirmation, type the password or provide confirmation.

Select the Networking tab. Under This connection uses the following items , select either Internet Protocol Version 4 (TCP/IPv4) or Internet Protocol Version 6 (TCP/IPv6) , and then select Properties .

To specify IPv4 IP address settings, do one of the following:

To get IP settings automatically using DHCP, select Obtain an IP address automatically , and then select OK .

To specify an IP address, select Use the following IP address , and then, in the IP address, Subnet mask , and Default gateway boxes, type the IP address settings.

To specify IPv6 IP address settings, do one of the following:

To get IP settings automatically using DHCP, select Obtain an IPv6 address automatically , and then select OK .

To specify an IP address, select Use the following IPv6 address , and then, in the IPv6 address, Subnet prefix length , and Default gateway boxes, type the IP address settings.

To specify DNS server address settings, do one of the following:

To get a DNS server address automatically using DHCP, select Obtain DNS server address automatically , and then select OK .

To specify a DNS server address, select Use the following DNS server addresses , and then, in the Preferred DNS server and Alternate DNS server boxes, type the addresses of the primary and secondary DNS servers.

To change advanced DNS, WINS, and IP settings, select Advanced .

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Static IP Address Assignment - Does Not Work

Two of my PC's are identical PC Sticks to which I need to sometimes remote in using their names and sometimes I need to remote into them using their IP addresses. They both run Win10 Pro, remote desktop is enabled and I am easily able to remote into both of them and take control. However, these two somehow act differently when static IP is being assigned.

PC "A" is fine. I go through the process of assigning a static IP address, do an ipconfig, and I see the assigned address, and I am able to remote into it using that static IP address. PC "B" does not respond in that way. If I vary the IP address from what is automatically assigned to it, the last number field in the address, and if it is different from the number assigned automatically, it doesn't take - in other words when I do an ipconfig I continue seeing the old IP address that was automatically assigned to it.

I have looked over various settings and compared them and the two PC's are identical in every aspect, including the WiFi driver hardware, driver, driver version and driver date. What do you think may be the problem that keeps the static IP address from being assigned?

Thanks in advance.

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After setting a fixed IP address, what do you see when you reopen the adapter's IP settings? Do you see the adress you set or did it flip back to DHCP?

Maybe your account has insufficient privileges to modify the relevant registry entries. You can find them here:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces

Your current network adapters are found here:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4d36e972-e325-11ce-bfc1-08002be10318}

The NetCfgInstanceId corresponds to the key name under the Interfaces key.

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Hi Fredrick.

An automatic IP assignment is: 192.168.0.17

If I change that to 192.168.0.18 then ipconfig reports Autoconfiguration IPV4 169.254.240.70.

If I set the IP to be automatically assigned, then ipconfig reports IPV4 Address 192.168.0.17 again.

I have looked at the HKEY_stuff and they are way ver my head. I must be doing something wrong in between these two exact - I mean exact computers.

Thanks for the input.

SOLVED THE PROBLEM.

Considering identical computers and identical software, I finally powered the router down and powered it back up, and the problem went away.

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Error message when you try to set an IP address on a network adapter

• 3 contributors

This article provides a solution to an error that occurs when you try to set an IP address on a network adapter.

Applies to:   Windows Server 2012 Original KB number:   269155

When you try to set the IP address on a network adapter, you may receive the following error message:

The IP address XXX.XXX.XXX.XXX you have entered for this network adapter is already assigned to another adapter Name of adapter . Name of adapter is hidden from the network and Dial-up Connections folder because it is not physically in the computer or is a legacy adapter that is not working. If the same address is assigned to both adapters and they become active, only one of them will use this address. This may result in incorrect system configuration. Do you want to enter a different IP address for this adapter in the list of IP addresses in the advanced dialog box?

In this error message, the placeholder XXX.XXX.XXX.XXX represents the IP address that you are trying to set, and the placeholder Name of adapter represents the name of the network adapter that is present in the registry but hidden in Device Manager.

If you select Yes , you see the TCP/IP properties. Then, you can change the IP address to something different for the currently displayed network adapter in Device Manager. If you select No , the IP address is assigned to the network adapter. Notice that if you select No , the selected network adapter in Device Manager and the ghosted network adapter have the same IP address. In most cases, this causes no problems, because the driver is disabled.

On a computer that is running Windows Server 2008, you receive the following error message:

Cannot rename this connection. A connection with the name you specified already exists. Specify a different name.

This issue occurs because a network adapter with the same IP address is in the registry but is hidden in Device Manager. It can occur when you move a network adapter from one PCI slot to another PCI slot.

To resolve this problem, uninstall the ghosted network adapter from the registry. To do it, use one of the following methods:

• In Windows 8 or in Windows Server 2012, use the Search charm to search for cmd, and then tap or select Command Prompt.
• In earlier versions of Windows, select Start , select Run , type cmd.exe, and then press Enter .
• Type set devmgr_show_nonpresent_devices=1 , and then press Enter.
• Type start devmgmt.msc , and then press Enter.
• Select View , and then select Show Hidden Devices .
• Expand the Network Adapters tree.
• Right-click the dimmed network adapter, and then select Uninstall .

The DevCon utility is a command prompt utility that acts as an alternative to Device Manager. When you use DevCon, you can enable, disable, restart, update, remove, and query individual devices or groups of devices. To use DevCon, follow these steps:

Download the DevCon tool in Windows Device Console (Devcon.exe) .

Unpack the 32-bit or 64-bit DevCon tool binary to a local folder.

Select Start , select Run , type cmd, and then press Enter . If you're running Windows 8 or Windows Server 2012, use the Search charm to search for cmd.

Type cd <path_to_binaries> to move to where devcon.exe is located.

Run the following command to list all network adapters that are present:

Run the following command to retrieve all network adapters:

For those network adapters that aren't listed in the result of the first command, note the instance ID of those items, and then run the following command for each instance ID:

More information

When you receive the error message that is mentioned in the Symptoms section, the hidden or ghosted network adapter doesn't appear in Device Manager unless you follow the steps in this article. Device Manager does show a network adapter, and the network adapter typically has #2 appended to its name.

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Networking Concepts

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Dynamic Address Assignment

In the early days of the Internet, every host was assigned a unique IP address that was hard-coded into its configuration files. As the Internet grew in popularity among less technically savvy users, and diskless devices became prevalent, it became impractical to manually change the IP address of each machine every time you changed networks. (This became even more important when laptops and wireless networking became commonplace.)

To solve this problem, a number of protocols were developed culminating in the modern Dynamic Host Configuration Protocol (DHCP) standard, IPv6 neighbor discovery, DHCPv6, and link-local addressing. These protocols are described further in the sections that follow.

Dynamic Host Configuration Protocol (DHCP) and DHCPv6

In IPv4, DHCP provides a means for a computer or other device to ask a central server for an IPv4 address suitable for use on the network. Depending on the server, the IPv4 address can be either assigned randomly from a pool of available addresses or assigned manually by an administrator based on the requesting host’s MAC address.

In IPv6, the DHCPv6 protocol can optionally be used in a similar way, though IPv6 addresses can also be acquired through stateless address autoconfiguration, as described in the next section.

A DHCP server can also provide additional information needed for routing—the subnet mask and router address, for example—as well as domain name servers, directory servers, and other arbitrary data as defined by various extensions to the protocol.

The way DHCP works (at a high level) can be summarized as follows: the client broadcasts a request for an IPv4 address. After the server assigns one, the client is said to own a “lease” for that IP address that lasts a particular period of time. The client may renew that lease at any time until the expiration date. The server is not obligated to agree to extend the lease, but most servers do.

Generally speaking, if the client is connecting for the first time, this exchange is a series of broadcast UDP packets. If the client is renewing an existing (still valid) IP lease, the exchange can be sent with unicast UDP packets (at the client’s discretion).

The client may continue to send requests for additional data (DNS servers, network volume mounts, Active Directory domain controllers, and so on) by sending requests with extra options and waiting for the reply.

Neighbor Discovery and IPv6 Address Assignment

The IPv6 protocol (built on top of ICMPv6) provides built-in support for neighbor discovery. Neighbor discovery serves two purposes:

Discovering the hardware address of other hosts on the same physical network. This takes the place of ARP.

Stateless address autoconfiguration (SLAAC). This is an alternative to much of the functionality provided by DHCP.

When a host first connects to a network, it initially uses a self-assigned IPv6 address, which is globally unique by design. This address allows the host to send neighbor discovery requests to solicit a router. The router then provides the same sort of information that would appear in a DHCP offer under IPv4—the IP address of the router, the network prefix (which is roughly comparable to the network address and subnet mask), the IP addresses of DNS servers, and so on. From there, the client can construct a routable IPv6 address to use when communicating with the public Internet.

In addition to the self-assigned link-local address, each host constructs at least two additional IPv6 addresses: a permanent address and a privacy address.

The host constructs its permanent address based on the host’s physical (link-layer) address. This results in an address whose host part generally remains unchanged as the computer moves from network to network. This address is intended to provide a consistent location where other hosts on the network can send data to the host in question.

Additionally, each host constructs one or more privacy addresses. These addresses are generated randomly and change over time. Thus, they are not tied to a particular piece of hardware. By default, outgoing connections are sent using a privacy address.

Link-Local Addressing and Bonjour

Domain names are a great solution for permanent servers, but they cost money and require a technically savvy user to configure them. Also, for non-public servers, a globally published domain name makes little sense. Further, standard domain names are fairly challenging to use with dynamic IP addresses assigned by DHCP servers and similar schemes. With an increasing number of dynamically configured networks, another solution was needed.

To provide an alternative, OS X and iOS support Bonjour, an implementation of zero-configuration networking. Bonjour consists of three parts:

Link-local addresses in IPv4—A means for self-assignment of IP addresses in the absence of a DHCP server or other means of IP address assignment. (Link local address assignment is built in to the IPv6 protocol itself, and thus there is no need for Bonjour to duplicate this functionality.)

Multicast DNS—A technology for providing DNS resolution when an infrastructure server is not present or when local, unmanaged names are more convenient.

DNS Service Discovery—A means of registering and discovering services.

The combination of these technologies allows multiple hosts on the same physical network to advertise services and discover one another without the need for a permanent DNS infrastructure.

Typically, Bonjour uses multicast DNS to send a DNS Service Discovery query to every machine on a local network, asking whether any of them provide a particular service. Each machine that provides the requested service then sends a message back to the original machine. The result is that those machines can discover and use services provided by other machines on the network without needing to tell each machine about those services ahead of time.

DNS Service Discovery can be combined with link-local addresses to allow zero-configuration networking—discovering networked devices even if you have no network infrastructure configured at all. For example, you might share files between two computers over an ad hoc wireless network.

The way link-local addresses work is fairly straightforward.

For IPv4, if a host fails to obtain an address from a DHCP server, the computer chooses a random IP address in a particular range ( 169.254.*.* ), then uses a link-specific protocol (such as ARP) to ask who has that address. If another machine responds, it tries a different address. If no machine responds, the host claims that number as its own.

For IPv6, every interface has a link-local address within the fe80:: prefix, regardless of whether the interface has another address assigned through SLAAC, DHCPv6, or other mechanisms. This address is computed based on the interface’s hardware address if the interface has one (an Ethernet MAC address, for example). If it does not, the address is chosen randomly within that prefix, and SLAAC’s duplicate address detection protocol is used to guarantee uniqueness. See RFC 4862 for details.

Because these IP addresses are all in the same subnet, every host with a link-local address on a given local area network can talk to every other host without going through a router. Thus, they can all discover one another without the need for any additional infrastructure.

Copyright © 2012 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2012-07-19

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Technical Tip: IPv4 address exhaustion in ike mode-cfg

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