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The Illustrated Network by Walter Goralski

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CHAPTER 7 Internet Control Message Protocol

What You Will Learn

In this chapter, you will learn about ICMP messages, their types, and (in many cases) the codes used in each type. We’ll look at which ICMP messages are routinely blocked at firewalls and which are essential for proper device operation.

You will learn about the common ping utility for determining device accessibility (“reachability”) on an IP network. We’ll discuss the mechanics of both ping and traceroute, and use several ping examples to illustrate ICMP on the network.

The only function of the IP layer is to provide addressing for and route the IP packet. That’s all. Once an IP packet has been dealt with, the IP layer just looks for the next packet. But IP is a connectionless, ...

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