2.6 IP Unnumbered
2.6.1 Using IP unnumbered
Throughout this chapter, you have seen ways to maximize an organization's use of IP addresses. In previous sections, you learned that you could avoid wasting an entire subnet on the point-to-point serial links by using VLSM, or use private addresses instead. Neither technique can be supported by classful routing protocols, such as the popular RIPv1 and IGRP. Fortunately, the Cisco IOS offers a third option for efficiently addressing serial links: IP unnumbered.

When a serial interface is configured for IP unnumbered, it borrows the IP address of another interface (usually a LAN interface or loopback interface) and therefore does not need its own address. 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 your network runs RIPv1 or IGRP, IP unnumbered may be the only solution to maximize your addresses.

RTA's S1 (168.71.5.1) and RTB's S1 (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 it is a point-to-point link, so there is no confusion about which device a packet is originating from or destined for. There are two ground rules for configuring IP unnumbered on an interface:
  • The interface is both serial and connected via 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.

Using IP unnumbered is not without its drawbacks, which include the following:

  • You cannot use ping to determine whether the interface is up because the interface has no IP address.
  • You cannot boot from a network IOS image over an unnumbered serial interface.
  • You cannot support IP security options on an unnumbered interface.