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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)
                9  ICMP Concepts and General Operation

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ICMP General Operation
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ICMP Message Creation and Processing Conventions and Rules
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ICMP Message Classes, Types and Codes
(Page 2 of 3)

ICMP Message Types

Each individual kind of message in ICMP is given its own unique Type value, which is put into the field of that name in the ICMP common message format. This field is 8 bits wide, so a theoretical maximum of 256 message types can be defined. A separate set of Type values is maintained for each of ICMPv4 and ICMPv6.

In ICMPv4, Type values were assigned sequentially, to both error and informational messages, on a “first come, first served” basis (sort of) so one cannot tell just by the Type value what type of message each is. One minor improvement made in ICMPv6 was that the message types were separated. In IPv6, error messages have Type values from 0 to 127, and informational messages have values from 128 to 255. Of course, only some of the Type values are currently defined.

Key Concept: A total of 256 different possible message types can be defined for each of ICMPv4 and ICMPv6. The Type field that appears in the header of each message specifies the kind of ICMP message. In ICMPv4 there is no relationship between Type value and message type; in ICMPv6 error messages have a Type value of 0 to 127, informational messages 128 to 255.


ICMP Message Codes

The message type indicates the general purpose of each kind of ICMP message. ICMP also provides an additional level of detail within each message type in the form of a Code field, which is also 8 bits. You can consider this field as a message “subtype”. Thus, each message type can have up to 256 subtypes that are more detailed subdivisions of the message's overall functionality. A good example is the Destination Unreachable message, which is generated when a datagram cannot be delivered. In this message type, the Code value provides more information on exactly why the delivery was not possible.


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ICMP Message Creation and Processing Conventions and Rules
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Version 3.0 - Version Date: September 20, 2005

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