Routable and routed protocols

A protocol is a set of rules that determines how computers communicate with each other across networks. Computers communicate with one another by exchanging data messages. To accept and act on these messages, computers must have definitions of how a message is interpreted. Examples of messages include those establishing a connection to a remote machine, e-mail messages, and files transferred over a network.

A protocol describes the following:

• The format that a message must conform to
• The way in which computers must exchange a message within the context of a particular activity

A routed protocol allows the router to forward data between nodes on different networks. In order for a protocol to be routable, it must provide the ability to assign a network number and a host number to each individual device. Some protocols, such as IPX, require only a network number because these protocols use the host's MAC address for the host number. Other protocols, such as IP, require a complete address consisting of a network portion and a host portion. These protocols also require a network mask in order to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask.

The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit. If this grouping were not allowed, each host would have to be mapped individually for routing. According to the Internet Software Consortium, this would not be possible with the 162,128,000 hosts that are currently on the Internet.