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s you know, to successfully troubleshoot network problems,

it is important to have a good understanding of how network components,
including PCs and servers, communicate with each other. Without this basic
knowledge, troubleshooting a network problem is like trying to read a book
in a foreign language. The information is there, but it just isn't comprehen-
sible. Although the model discussed in Chapter 1 provides the method of
retrieving all of the necessary information, the data is useless without an
understanding of the information presented.

This chapter is a review of the protocols that are used by Layers 2, 3, and

4 of the OSI model. We briefly review the seven layers of the OSI model, and
then discuss how they communicate with one another. We then discuss
Layer 2 and Layer 3 protocols.

The OSI Reference Model

his section is a review of the OSI model, which was originally dis-

cussed in

CCNA: Cisco Certified Network Associate Study Guide,

2nd ed.,

by Todd Lammle (ISBN 0-7821-2647-2, Sybex, 2000). The OSI model (or
the Open Systems Interconnection reference model) is the template used
to design applications or protocols that allow non-homogenous computers
or networks to communicate with one another. The ISO (International
Organization for Standardization) developed the OSI model.

The OSI model consists of seven layers. Each layer communicates directly

with its adjacent layers, as well as with the corresponding layer of the desti-
nation system (depicted in Figure 2.1). Communication between layers
facilitates the transfer of data up and down the OSI model. Communication
between the corresponding layers of the source system and the destination
system enables two heterogeneous networks or computers to understand
each other.

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