Address Mask Request or Address Mask Reply
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Addresses
The address of the source in an address mask request message
will be the destination of the address mask reply message.
To form an address mask reply message, the source address of
the request becomes the destination address of the reply,
the source address of the reply is set to the replier's
address, the type code changed to AM2, the address mask
value inserted into the Address Mask field, and the checksum
recomputed. However, if the source address in the request
message is zero, then the destination address for the reply
message should denote a broadcast.
ICMP Fields:
Type
AM1 for address mask request message
AM2 for address mask reply message
Code
0 for address mask request message
0 for address mask reply message
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP
Type. For computing the checksum, the checksum field should
be zero. This checksum may be replaced in the future.
Identifier
An identifier to aid in matching requests and replies, may
be zero.
Sequence Number
A sequence number to aid in matching requests and replies,
may be zero.
Address Mask
A 32-bit mask.
Description
A gateway receiving an address mask request should return it
with the address mask field set to the 32-bit mask of the bits
identifying the subnet and network, for the subnet on which the
request was received.
If the requesting host does not know its own IP address, it may
leave the source field zero; the reply should then be
broadcast. However, this approach should be avoided if at all
possible, since it increases the superfluous broadcast load on
the network. Even when the replies are broadcast, since there
is only one possible address mask for a subnet, there is no
need to match requests with replies. The "Identifier" and
"Sequence Number" fields can be ignored.
Type AM1 may be received from a gateway or a host.
Type AM2 may be received from a gateway, or a host acting in
lieu of a gateway.