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Appendix I. Address Mask ICMP Connected: An Internet Encyclopedia
Appendix I. Address Mask ICMP

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Appendix I. Address Mask ICMP

Appendix I. Address Mask ICMP

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.

Related Information


Next: Appendix II. Examples

Connected: An Internet Encyclopedia
Appendix I. Address Mask ICMP

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