To connect to a TCP/IP network, a computer must have at least one network interface, and 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. If you add a second network interface to a host and disable routing, the host is then considered a multihomed host -- a machine with more than one network interface that does not run routing protocols or forward IP packets.