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Describing Protocols by Referencing the OSI Layers

Even today , networking documents often describe TCP/IP protocols and standards by referencing
OSI layers, both by layer number and layer name. For example, a common description
of a LAN switch is “Layer 2 switch,” with “Layer 2” referring to OSI layer 2. Because
OSI did have a well-defined set of functions associated with each of its seven layers, if you
know those functions, you can understand what people mean when they refer to a product
or function by its OSI layer.

For another example, TCP/IP’s original Internet layer, as implemented mainly by IP, equates
most directly to the OSI network layer. So, most people say that IP is a network layer protocol,
or a Layer 3 protocol, using OSI terminology and numbers for the layer. Of course,
if you numbered the TCP/IP model, starting at the bottom, IP would be either Layer 2 or 3,
depending on what version of the TCP/IP model you care to use. However, even though IP
is a TCP/IP protocol, everyone uses the OSI model layer names and numbers when describing
IP or any other protocol for that matter.

The claim that a particular TCP/IP layer is similar to a particular OSI layer is a general
comparison, but not a detailed comparison. The comparison is a little like comparing
a car to a truck: Both can get you from point A to point B, but they have many specific
differences, like the truck having a truck bed in which to carry cargo. Similarly, both
the OSI and TCP/IP network layers define logical addressing and routing. However, the
addresses have a different size, and the routing logic even works differently. So the comparison
of OSI layers to other protocol models is a general comparison of major goals,
and not a comparison of the specific methods.

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