Network interfaces must be flexible enough to allow you to connect and disconnect from a subnet. Once a network interface^[1] is configured, only one simple step is required to begin using the interface.

Interfaces must be activated, or enabled, before communications among computers can begin. The simplest way to activate an interface is to use netcfg in one of the following ways:

1. Click your interface and then click Activate
2. Set the interface to automatically start-up at boot time

An even simpler method is the command line utility, ifup. To activate the interface eth0, type ifup eth0 at a command prompt. If you have configured the connection properly and the network is functioning correctly, then the interface will activate. From there, you can begin using network applications like your Web browser or FTP and Telnet clients.

When you wish to disable an interface, you can use the same netcfg method used to enable an interface. For example, if you have a modem using interface ppp0 and you need to disable the interface to use a telephone on the same line, open netcfg to the Interfaces screen, select the ppp0 interface, and click Deactivate.
Alternatively, you can just type ifdown ppp0 at the command prompt. The ifdown command is the opposite of ifup, as you might have guessed. Use it whenever you wish to disable an interface. The next lesson shows you how to use the Host Table to look up network IP addresses and host names.

Before moving on to the next lesson, click the Quiz link below to review configuration and activation.
Enabling Disabling - Quiz

[1]Interface: A communications system used to connect a Linux machine to a network.