If configured with two or more network adapters using the TCP/IP protocol, Windows XP has the ability to serve as a router, and will pass traffic between the two networks. You may want to do this if you have a test network that you want to keep isolated from your office LAN.
Here's how routing works: If Windows receives a data packet whose IP address matches no network interface in the computer, it will try to forward the packet according to its routing table. In general:
If the destination address belongs to the subnet of one of the installed network adapters, Windows will send the packet out that adapter (in the context of routing, it's more often called an interface).
If the destination address belongs to a subnet ...