3.2 Default Routing
3.2.1 Default routing overview
It is not feasible, or even desirable, for every router to maintain routes to every possible destination. Instead, routers can keep a default route, or a gateway of last resort. Default routes are used when the router can not match a destination network with a more specific entry in the routing table; thus, the gateway of last resort. In effect, the router uses the default route to hand off to another router. The other router must have either a route to that destination or its own default route to a third router. If it is a default route to a third router, that router must have either the route to the destination or another default route, and so on. Eventually, the packet should be routed to a router that actually has a route to the destination.

A key scalability feature is that default routes keep routing tables as lean as possible. They make it possible for routers to forward packets destined to any Internet host without having to maintain a table entry for every Internet network. Default routes can be statically entered by an administrator or dynamically learned via a routing protocol.

Default routing begins with the administrator. Before routers can dynamically exchange default information, an administrator must configure at least one router with a default route. An administrator can use two very different commands to statically configure default routes: ip route 0.0.0.0 0.0.0.0 and ip default-network.

The following sections explore these two methods in detail.