The Seven Layers of the OSI Reference Model

OSI sounds like the name of a top-secret government agency you hear about only in Tom Clancy novels. What it really stands for in the networking world is Open Systems Interconnection, as in the Open Systems Interconnection Reference Model, affectionately known as the OSI model.

The OSI model breaks the various aspects of a computer network into seven distinct layers. These layers are kind of like the layers of an onion: Each successive layer envelops the layer beneath it, hiding its details from the levels above. The OSI model is also like an onion in that if you start to peel it apart to have a look inside, you’re bound to shed a few tears.

The OSI model is not a networking standard in the same sense that Ethernet and Token Ring are networking standards. Rather, the OSI model is a framework into which the various networking standards can fit. The OSI model specifies what aspects of a network’s operation can be addressed by various network standards. So, in a sense, the OSI model is sort of a standard of standards.

The first three layers are sometimes called the lower layers. They deal with the mechanics of how information is sent from one computer to another over a network. Layers 4 through 7 are sometimes called the upper layers. They deal with how applications programs relate to the network through application programming interfaces.