Network Router Enablement

It may seem strange, but when you first enable your network router and configure IP addresses for its interfaces, the router will not actually route data. First, you must "tell" the router it is allowed to do so by following these steps:

Router2>enable
Password:
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface fastEthernet 0/0
Router2(config-if)#ip address 192.168.1.240 255.255.255.0
Router2(config-if)#no shutdown
Router2(config-if)#exit
Router2(config)#interface fastEthernet 0/1
Router2(config-if)#ip address 192.168.5.1 255.255.255.0
Router2(config-if)#no shutdown
Router2(config-if)#exit
Router2(config)#ip routing
Router2(config)#exit

The first part of this process sets up your network interfaces. The key to allowing your router to route traffic between its configured interfaces is found in the ip routing command, which turns on the routing processes.

Unlike with switches, where every interface (or port) on a switch is enabled by default, with routers, you have to specifically tell a router to enable an interface, by specifying the no shutdown command when you are in Interface Configuration mode.

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Source:http://www.dummies.com/how-to/content/network-router-enablement.html