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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Protocol Layering
The Internet Protocol (IP) divides the tasks necessary to route and forward packets across networks by
using a layered approach. Each layer has clearly defined tasks, protocol, and interfaces for
communicating with adjacent layers, but the exact way these tasks are accomplished is left to
individual software designers. The Open Systems Interconnect (OSI) seven-layer model has been
adopted as the reference for the description of modern networking, including the Internet.
A diagram of the OSI model is shown below (note that this is not a complete listing of the protocols
contained within each layer of the model):
Figure 5-11. OSI Seven Layer Network Model
Each layer is a distinct set of programs executing a distinct set of protocols designed to accomplish
some necessary tasks. They are separated from the other layers within the same system or network,
but must communicate and interoperate. This requires very well-defined and well-known methods for
transferring messages and data. This is accomplished through the protocol stack.
Protocol layering as simply a tool for visualizing the organization of the necessary software and
hardware in a network. In this view, Layer 2 represents switching and Layer 3 represents routing.
Protocol layering is actually a set of guidelines used in writing programs and designing hardware that
delegate network functions and allow the layers to communicate. How these layers communicate within
a stack (for example, within a given computer) is left to the operating system programmers.
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