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ICMPv4 Address Mask Request and Reply Messages
(Page 2 of 2)
ICMPv4 Address Mask Request and Address Mask Reply Message Format
The Address Mask Request and
Address Mask Reply, like some other request/reply pairs, have
the same basic formatthe host creates the request with all fields
filled in except the subnet mask value itself, and the router supplies
the mask and sends the reply back to the host. The format is described
in Table 100
and Figure 150.
Table 100: ICMPv4 Address Mask Request and Address Mask Reply Message Format
Field
Name
|
Size (bytes)
|
Description
|
Type
|
1
|
Type: Identifies
the ICMP message type. For Address Mask Request messages the
value is 17; for Address Mask Reply messages, it is 18.
|
Code
|
1
|
Code:
Not used for either message type; set to 0.
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Checksum
|
2
|
Checksum: 16-bit
checksum field for the ICMP header, as described in the
topic on the ICMP common message format.
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Identifier
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2
|
Identifier:
An identification field that can be used to help in matching Address
Mask Request and Address Mask Reply messages.
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Sequence
Number
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2
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Sequence Number:
A sequence number to help in matching Address Mask Request and
Address Mask Reply messages.
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Address
Mask
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4
|
Address
Mask: The subnet mask for the local network, filled in by the
router in the Address Mask Reply message.
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The Identifier and
Sequence Number fields can be used to match up requests and replies,
as they are for Echo and Echo Reply messages. However,
a host won't normally send multiple requests for subnet masks the way
it might send Echo messages for testing. For this reason, the
Identifier and Sequence Number fields may be ignored by
some implementations.
Figure 150: ICMPv4 Address Mask Request and Address Mask Reply Message Format
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Use of Address Mask Request and Address Mask Reply Messages
The use of Address Mask Request
and Address Mask Reply messages is optional, just as the router
discovery described in the previous topic is. Other methods besides
these messages or manual configuration may be used to tell a host what
subnet mask to use. Again, a common alternative to ICMP for this is
a host configuration protocol like the TCP/IP Dynamic
Host Configuration Protocol (DHCP). Routers
do have to be able to respond to Address Mask Requests for hosts
that choose to send them.
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Version 3.0 - Version Date: September 20, 2005
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