In order to connect to the network, a computer must have at least one network interface. Each network interface must have its own unique IP address. The IP address that you give to a host is assigned to its network interface, sometimes referred to as the primary network interface. If you add a second network interface to a machine, the machine must have its own unique IP number. When you add a second network interface, the machine changes to a router. See Configuring Routers for an explanation. If you add a second network interface to a host and you disable routing, the host is then considered a multihomed host.
Each network interface has a device name, device driver, and an associated device file in the /devices directory. The network interface might have a device name, such as le0 or smc0, device names for two commonly used Ethernet interfaces.
This book assumes that your machines have Ethernet network interfaces. If you plan to use different network media, refer to the manuals that come with the network interface for configuration information.