In order to connect to the network, a computer must have at least one network interface, as explained in "Network Interfaces". 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, it must have its own unique IP number. Adding a second network interface changes the function of a machine from a host to a router, as explained in Chapter 5, Configuring Routers. If you add a second network interface to a host and disable routing, the host is then considered a multihomed host.
Each network interface has a device name, device driver, and 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 came with the network interface for configuration information.