Computer networks allow messages to be exchanged between computers in different parts of the world. These messages may contain e-mails, requests for web pages, or the contents of a web page. When traveling from the source computer to the destination computer, messages typically pass through a number of other computers on the network. Routing is the function of choosing which computers a message should pass through on its way from source to destination. Some computers in the network, called routers, exist only to route messages onward in the network. Routing must be performed in large networks such as the Internet as well as in smaller networks such as within a university.

When a message is received from the network, the computer that receives the message—unless it is the intended recipient—must perform a routing action. Routing the message means choosing which of the connected computers should receive...