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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
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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Version 3.0 - Version Date: September 20, 2005
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