Determining the gateway of last resort

Determining the gateway of last resort 
9.1.2 This page will teach students how to configure a gateway of last resort and why it is used.
It is not feasible, or even desirable, for a router to maintain routes to every possible destination. Instead, routers keep a default route, or a gateway of last resort. Default routes are used when the router is unable to match a destination network with a more specific entry in the routing table. The router uses this default route to reach the gateway of last resort in an effort to forward the packet. 
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 using a routing protocol.
Default routing begins with the administrator. Before routers can dynamically exchange information, an administrator must configure at least one router with a default route. Depending on the desired results, an administrator can use either of the following commands to statically configure a default route: 

ip default-network

or

ip route 0.0.0.0 0.0.0.0

The ip default-network command is used to establish a default route in networks that use dynamic routing protocols. The ip default-network command is classful, which means if the router has a route to the subnet indicated by this command, it installs the route to the major net. The ip default-network command must be issued using the major net, in order to flag the candidate default route.
The global command ip default-network 192.168.17.0 defines the Class C network 192.168.17.0 as the destination path for packets that have no routing table entries. Any routes to a network configured with ip default-network will be flagged as a candidate for the default route. 
The ip route 0.0.0.0/0 command can also be used to configure a default route. 

Router(config)#ip route prefix mask {address 1 interface } [distance ]

After configuring a default route or default network, the command show ip route will show the following: 

Gateway of last resort is 172.16.1.2 to network 0.0.0.0

The Lab Activity will show students how to configure a gateway of last resort.
The next page explains how path determination occurs.