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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)

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ICMP Common Message Format and Data Encapsulation
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ICMP Version 4 (ICMPv4) Error Message Types and Formats
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ICMP Message Types and Formats

The Internet Control Message Protocol (ICMP) provides a mechanism for error reporting and the exchange of important information between IP devices. Many different message types exist within ICMP, which have certain common attributes that we saw in the section discussing ICMP generalities. Beyond these commonalities, the details of how ICMP messages are created and used diverge based on the specific message type, so it's time to look at these and see how they work.

In this section I provide specific details on the most commonly-seen ICMP message types, including a description of both their use and their specific message formats. There are 20 different message types currently covered, so they have been divided into four subsections for better organization. These correspond to the four combinations of ICMP version (ICMPv4 and ICMPv6) and message class (error and informational).

Related Information: For a cross-reference of message types and corresponding ICMP Type field values, see the general topic on ICMP message classes and types. For more information on the “common” ICMP message format fields (Type, Code and Checksum) and incorporation of original IP datagram information in error messages, see the topic on ICMP Common Message Format and Data Encapsulation.


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