2.7 Using IP Unnumbered
2.7.1 Understanding IP unnumbered
IP unnumbered refers to using the network or subnet address of a local LAN interface as the router's network or subnetwork address for a point-to-point serial link. The term point-to-point means that only two devices are on the link, as is the case on a T1 connection between two routers or a point-to-point subinterface to a Frame Relay network. Normally, a serial link has its own unique network or subnetwork address. IP unnumbered allows a network administrator to conserve network or subnetwork addresses. IP unnumbered is especially valuable for networks running IP routing protocols such as RIP V1 and IGRP, which do not support VLSM. Without VLSM support, a network must use the same subnet mask on its serial (WAN) interfaces as it does on its LAN interfaces. For a company using an 24-bit (255.255.255.0) mask on a Class B address, this means applying a subnet capable of supporting 254 hosts on a WAN link that never has more than two hosts.
Lab Activity
  In this lab, you will learn how improper IP addressing can have a negative impact on a network and the use of IP Unnumbered on a point to point serial link to save IP addresses.

One of the reasons IP unnumbered is possible is that with only two hosts on any serial link --- when the routers are at either end --- there is no confusion about which device a packet is originating from or destined for. The following are a few rules that apply to using IP unnumbered:

  • The serial interface must be a point-to-point link.
  • You must use the same major network with the same mask on both sides of the WAN link.

           or

  • You must use different major nets with no subnetting on both sides of the WAN link.

These rules are discussed in more detail in the following sections.

Configuring IP Unnumbered on Serial Interfaces

Configuring IP unnumbered for RIP and IGRP is done in the same way as explained previously. The excerpts from the configurations of RouterA and RouterC show how to configure IP unnumbered on a serial interface. (see RouterA#show running-config command output).

The show interface commands from RouterA and RouterC show that IP unnumbered is now in use on both serial interfaces. (see RouterA#show interface serial 1 command output).