ICMP Message Classes, Types, and Codes

ICMP messages are used to allow the communication of different types of information between IP devices on an internetwork. The messages themselves are used for a wide variety of purposes, and they are organized into general categories as well as numerous specific types and subtypes.

ICMP Message Classes

At the highest level, ICMP messages are divided into two classes:

Error Messages These messages are used to provide feedback to a source device about an error that has occurred. They are typically generated specifically in response to some sort of action, usually the transmission of a datagram, as shown in the example in Figure 31-1. Errors are usually related to the structure or content of a datagram or to problem ...

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