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 TCP/IP 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.
| Layer | Name | Description |
|---|---|---|
| 1 | Physical | Governs the layout of cables and devices such as repeaters and hubs. |
| 2 | Data Link | Provides MAC addresses to uniquely identify network nodes and a means for data to be sent over the Physical layer in the form of packets. Bridges and switches are layer 2 devices. |
| 3 | Network | Handles routing of data across network segments. |
| 4 | Transport | Provides for reliable delivery of packets. |
| 5 | Session | Establishes sessions between network applications. |
| 6 | Presentation | Converts data so that systems that use different data formats can exchange information. |
| 7 | Application | Allows applications to request network services. |
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 application software can relate to the network through application programming interfaces.
The seven layers of the OSI model are a somewhat idealized view of how networking protocols should work. In the real world, actual networking protocols don’t follow the OSI model to the letter. The real world is always messier than ideal. Still, the OSI model provides a convenient — if not completely accurate — conceptual picture of how networking works.