What is a network protocol?

A network protocol is, like all other protocols (i.e. human protocols or other machine protocols), a set of predetermined rules that allow two entities that have never met to communicate. A handshake is a protocol (which is why we talk about modems handshaking). Two computers set up to use the same protocol will know what messages to expect and how to respond. But two computers set up to use two different protocols will not be able to communicate even if they are on the same physical network and use the same cabling scheme. A computer set up to communicate using the IPX protocol will not be able to communicate with a computer set up to communicate using TCP/IP even if both are on the same physical Ethernet cable. They will never agree on which messages to receive, what they contain, or how to answer.

The word 'protocol' is often used loosely, without distinction as to whether it is a bona fide protocol such as TCP/IP or IPX, or simply a member of a larger protocol suite. HTTP is a protocol, but belongs to the TCP/IP protocol suite. In the world of TCP/IP, any three or four letter abbreviation ending with a 'P' (FTP, DHCP, UDP...) is usually a protocol.

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