Content

| 2.6 |  | IP Unnumbered |
| 2.6.1 |  | Using IP unnumbered |

This module has presented several ways to maximize the use of IP addresses in an organization. In previous sections, it was learned that wasting an entire subnet on the point-to-point serial links could be avoided by using VLSM, or use private addresses instead. Neither technique can be supported by classful routing protocols, such as the popular RIP v1 and IGRP. Fortunately, the Cisco IOS offers a third option for efficiently addressing serial links. This option is known as IP unnumbered.

When a serial interface is configured for IP unnumbered, it does not need its own address. This is because it borrows the IP address of another interface, usually a LAN interface or loopback interface. Not only does IP unnumbered avoid wasting addresses on point-to-point WAN links, but it also can be used with classful routing protocols, where VLSM and discontiguous subnets cannot. If the network runs RIP v1 or IGRP, IP unnumbered may be the only solution to maximize the addresses.

The RTA E0, 168.71.5.1, and RTB E0, 168.71.8.1, can communicate using TCP/IP over this serial link, even though they do not belong to the same IP network. This is possible because the serial link is a point-to-point link, so there is no confusion about which device a packet is originating from or destined for. In this case, the command

ip unnumbered e0 would be entered in serial 1 interface configuration mode on both RTA and RTB. There are two ground rules for configuring IP unnumbered on an interface:

- The interface is both serial and connected by way of a point-to-point link.
- The same major network with the same mask is used to address the LAN interfaces that lend their IP address on both sides of the WAN link.
  OR
  Different major networks with no subnetting are used to address the LAN interfaces on both sides of the WAN link.

There are certain drawbacks that come with using IP unnumbered:

- The use of

ping cannot determine whether the interface is up because the interface has no IP address.
- A network IOS image cannot boot over an unnumbered serial interface.
- IP security options cannot be supported on an unnumbered interface.

| Web Links Understanding and Configuring the

ip unnumbered Command

http://www.cisco.com/warp/public/701/20.html