7.2 Default Routing
7.2.6 Using IP classless

A simpler method for forcing a router to use a gateway of last resort for remote subnets that are part of the local domain is to configure the ip classless command. This command causes the router to ignore all aspects of address classes when making routing decisions. The following excerpt from the router C configuration shows how ip classless has been configured. 

!
ip classless
!

Note: The ip classless command is global. It affects all routing protocols in the router. It should be used with caution. A thorough discussion of all the implications of using this command is beyond the scope of this book. Please refer to your Cisco IOS documentation for more information.

In the output of the show ip route command on router C, you can see that, once again, router C has no knowledge of how it should route packets for 168.71.6.0. (see RouterC#show ip route 168.71.6.0 command output).

In the output from router C pinging 168.71.6.1, you can see that the ping is successful. (see RouterC#ping 168.71.6.1 command output).

As you can see, router C can ping a local domain subnet that it has no knowledge of, even though the local domain default route pointing to 10.0.0.0 (ip route 168.71.0.0 255.255.0.0 10.0.0.0) has not been configured. Remember that this was a required configuration parameter before the ip classless command was configured.