Determining the Gateway of Last Resort

Router(config)#ip default-network w.x.y.z	Sets network w.x.y.z to be the default route. All routes not in the routing table will be sent to this network
Router(config)#ip route 0.0.0.0 0.0.0.0 172.16.20.1	Specifies that all routes not in the routing table will be sent to 172.16.20.1

Note:

You must use the ip default-network command with IGRP. Although you can use it with EIGRP or RIP, it is not recommended. Use the ip route 0.0.0.0 0.0.0.0 command instead.

Routers that use the ip default-network command must have either a specific route to that network or a 0.0.0.0 /0 default route.