This chapter has presented several ways to maximize the use of IP addresses in an organization. In previous sections, you learned that you can avoid wasting an entire subnet on 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 software offers a third option for efficiently addressing serial linksIP unnumbered.
Using 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. Example 2-2 shows how to configure an unnumbered 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, whereas VLSM and discontiguous subnets cannot. If the network runs RIPv1 or IGRP, IP unnumbered might be the only solution to maximize the addresses.
Example 2-2 Configuring an IP Unnumbered Interface
RTA(config)#interface e0 RTA(config-if)#ip address 126.96.36.199 255.255.255.0 RTA(config-if)#interface s1 RTA(config-if)#ip unnumbered e0 RTB(config)#interface e0 RTB(config-if)#ip address 188.8.131.52 255.255.255.0 RTB(config-if)#interface s1 RTB(config-if)#ip unnumbered e0
RTA e0, 184.108.40.206, and RTB e0, 220.127.116.11, can communicate using TCP/IP over this serial link, even though they do not belong to the same IP network, as shown in Figure 2-16. 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 is entered in serial 1 interface configuration mode on both RTA and RTB. Configuring IP unnumbered on an interface has two ground rules:
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.
Different major networks with no subnetting are used to address the LAN interfaces on both sides of the WAN link.
Figure 2-16 IP Unnumbered Interfaces
Certain drawbacks 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.