Do you have questions about networking, VPN security or VoIP? Then visit Firewall.cx, one of the few websites recommended by Cisco Systems in its world class Cisco Academy program.
Almost all interfaces on a router are configurable with an IP address. Some interfaces, such as the Fast Ethernet you mentioned, can be configured with one or two IP addresses. This way, you can have one physical interface configured to support two logical networks.
For example, the Fast Ethernet interface can have IP addresses 192.168.0.1 and 192.168.1.1 assigned to it. This means both networks (192.168.0.0/24 & 192.168.1.0/24) can 'see' the router.
This was first published in July 2008