Configuring Router Interfaces

In order to get our router to do anything truly useful, we’ll need to configure its interfaces. Recall that on a Cisco 2501, we have one Ethernet interface and two serial interfaces. We’ll start off by configuring these interfaces with IP addresses, and follow up by configuring IPX addresses.

Ethernet and serial interfaces are configured from the interface level of global configuration mode. In order to configure interface Ethernet 0, we’ll need to access that interface.

cisco2501#config t
Enter configuration commands, one per line. End with CNTL/Z.
cisco2501(config)#int e0

Our next step will be assigning interface Ethernet 0 an IP address. This couldn’t be easier – we’ll simply use the ip address command.

cisco2501(config-if)#ip address
% Incomplete command.

After entering the ip address command along with the address we want to configure, we’re faced with the incomplete command message. Obviously that means that we’ve forgotten something. Use the question mark to figure out what that something is.

cisco2501(config-if)#ip address ?
A.B.C.D IP subnet mask

It’s now clear that we’re supposed to also add a subnet mask value following the IP address. In this case, we’ll use a mask of

cisco2501(config-if)#ip address

The address is now set, but even if you tried to ping it from another system, you wouldn’t receive a reply. Why not? Because even though it has been configured, the interface is still not “turned on”. In order to make the interface active, you will need to enter the no shutdown command.

cisco2501(config-if)#no shutdown

Assuming that you have a straight cable connecting the Ethernet 0 interface to a hub or switch, you should now be able to ping it from other systems.

Tip: As a best practice, issue the show interface command after configuring an interface to be sure that it is functioning correctly.

Configuring our router’s serial interfaces is really no different – access the serial interface that you wish to configure, and allocate an IP address and subnet mask using the same command. Remember that the no shutdown command will also need to be issued. The output below shows the configuration of IP address on interface serial 0.

cisco2501(config)#int serial 0
cisco2501(config-if)#ip address
cisco2501(config-if)#no shutdown

A variety of other properties can be configured for interfaces beyond their addresses and shutdown properties. To get a listing on the commands available, use the question mark from the router(config-if)# prompt.

It’s usually a good idea to also add a description to interfaces. For example, we could add a description to the serial 1 interface, mentioning that it connects the Toronto router to our Montreal location. This is also accomplished from the interface level of global configuration mode.

cisco2501(config)#int s1
cisco2501(config-if)#description WAN link Toronto to Montreal
cisco2501#show run
Building configuration...
Current configuration:
interface Serial1
description WAN link Toronto to Montreal
ip address

I once again truncated the output from the show run command, limiting it to the pertinent information about the serial 0 interface. Notice the description is now included in the interface section. This provides a quick and easy way to reference what a given interface connects to.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.