Conserving Subnets with IP Unnumbered
typically, a link between two routers requires a subnet. With classful routing protocols, such as RIP, this is problematic because you waste a multihost subnet for just two routers. Better solutions are to use VLSM and create small, two-host subnets (255.255.255.252 or /30 subnet mask) or to use the IOS //* unnumbered feature.
With IP unnumbered, you can save substantial address space by deploying router links without assigned subnets. This feature is applicable to point-to-point networks between router pairs, such as point-toj^ji^ line, frame relay, and ATM links.
Figure 1-10 illustrates the difference between standard addressing and IP unnumbered.
Figure 1-10 Standard Addressing Versus IP Unnumbered
Standard Addressing 172.16.3.0/24
172.16.3.2
IP Unnumbered
- No subnet assigned
in standard addressing, you assign a subnet to each router interface. For Figure 1-10, this means the interfaces EthernetO (E0) and SerialO (SO) on both routers are assigned specific subnets. One subnet, 172.16.3.0/24, exists only to connect the two routers—a waste of addresses. The subnet could be used more effectively; it could support a LAN with clients and servers, for example.
With IP unnumbered, the point-to-point serial interfaces have no assigned addresses and have no subnet between them. This would normally cause problems because a router uses the interface address as the source address for routing updates it sends out that interface. IP unnumbered resolves the problem by borrowing an address from one of the router's other interfaces (a LAN interface, for example) and using the borrowed address for the source address of routing updates it generates out of the unnumbered interface.
To configure ÏP unnumbered and designate the interface from which to borrow an address, use the ip unnumbered interface configuration command. The following example starts from enable mode:
Router#config terminal Router(config)#interface S0 Rcuter(config-intf)#ip unnumbered e0
The command ip unnumbered eO configures SerialO (sO) as an unnumbered interface and designates the address configured on EthernctO (eO) as the borrowed address (the source address for routing updates going out the unnumbered interface SerialO).
NOTE You should be familiar with configuring Cisco routers from the IOS command line. For a quick-start tutorial on navigating around IOS and entering commands, refer to Appendix 1i, "A Crash Course in Cisco IOS.H
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