rfc917.txt

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            request message is zero, then the destination address for
            the reply message should denote a broadcast.

      ICMP Fields:

         Type

            A1 for address format request message

            A2 for address format reply message

         Code

            0 for address format request message

            Width of subnet field, in bits, for address format reply
            message

         Checksum

            The checksum is the 16-bit one's complement of the one's




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RFC 917                                                     October 1984
Internet Subnets


            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 request and replies, may be
            zero.

         Sequence Number

            A sequence number to aid in matching request and replies,
            may be zero.

      Description

         A gateway receiving an address format request should return it
         with the Code field set to the number of bits of Subnet number
         in IP addresses for the network to which the datagram was
         addressed.  If the request was broadcast, the destination
         network is "this network".  The Subnet field width may be from
         0 to (31 - N), where N is the width in bits of the IP net
         number field (i.e., 8, 16, or 24).

         If the requesting host does not know its own IP address, it may
         leave the source field zero; the reply should then be
         broadcast.  Since there is only one possible address format for
         a network, there is no need to match requests with replies.
         However, this approach should be avoided if at all possible,
         since it increases the superfluous broadcast load on the
         network.

            Type A1 may be received from a gateway or a host.

            Type A2 may be received from a gateway, or a host acting in
            lieu of a gateway.













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RFC 917                                                     October 1984
Internet Subnets


II. Examples

   For these examples, we assume that the requesting host has  address
   36.40.0.123, that there is a gateway at 36.40.0.62, and that on
   network 36.0.0.0, an 8-bit wide subnet field is in use.

   First, suppose that broadcasting is allowed, and that 36.40.0.123
   knows  its own address.  It sends the following datagram:

      Source address:          36.40.0.123
      Destination address:     36.255.255.255
      Protocol:                ICMP = 1
      Type:                    Address Format Request = A1
      Code:                    0

   36.40.0.62 will hear the datagram, and should respond with this
   datagram:

      Source address:          36.40.0.62
      Destination address:     36.40.0.123
      Protocol:                ICMP = 1
      Type:                    Address Format Reply = A2
      Code:                    8

   For the following examples, assume that address 255.255.255.255
   denotes "broadcast to this physical network", as described in [6].

   The previous example is inefficient, because it potentially
   broadcasts  the request on many subnets.  The most efficient method,
   and the one we recommend, is for a host to first discover its own
   address (perhaps  using the "Reverse ARP" protocol described in [4]),
   and then to send  the ICMP request to 255.255.255.255:

      Source address:          36.40.0.123
      Destination address:     255.255.255.255
      Protocol:                ICMP = 1
      Type:                    Address Format Request = A1
      Code:                    0

   The gateway can then respond directly to the requesting host.

   Suppose that 36.40.0.123 is a diskless workstation, and does not know
   even its own host number.  It could send the following datagram:






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RFC 917                                                     October 1984
Internet Subnets


      Source address:          0.0.0.0
      Destination address:     255.255.255.255
      Protocol:                ICMP = 1
      Type:                    Address Format Request = A1
      Code:                    0

   36.40.0.62 will hear the datagram, and should respond with this
   datagram:

      Source address:          36.40.0.62
      Destination address:     36.40.255.255
      Protocol:                ICMP = 1
      Type:                    Address Format Reply = A2
      Code:                    8

   Note that the gateway uses the narrowest possible broadcast to reply
   (i.e., sending the reply to 36.255.255.255 would mean that it is
   transmitted on many subnets, not just the one on which it is needed.)
   Even so, the overuse of broadcasts presents an unnecessary load to
   all hosts on the subnet, and so we recommend that use of the
   "anonymous" (0.0.0.0) source address be kept to a minimum.

   If  broadcasting is not allowed, we assume that hosts have wired-in
   information about neighbor gateways; thus, 36.40.0.123 might send
   this datagram:

      Source address:          36.40.0.123
      Destination address:     36.40.0.62
      Protocol:                ICMP = 1
      Type:                    Address Format Request = A1
      Code:                    0

   36.40.0.62 should respond exactly as in the previous case.
















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RFC 917                                                     October 1984
Internet Subnets


Notes

   <1>  For example, some host have addresses assigned by concatenating
        their Class A network number with the low-order 24 bits of a
        48-bit Ethernet hardware address.

   <2>  Our discussion of Internet broadcasting is based on [6].

   <3>  If broadcasting is not supported, them presumably a host "knows"
        the address of a neighbor gateway, and should send the ICMP to
        that gateway.

   <4>  This is what was referred to earlier as the coexistence of
        transparent and explicit subnets on a single network.



































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RFC 917                                                     October 1984
Internet Subnets


References

   1.  D.R. Boggs, J.F. Shoch, E.A. Taft, and R.M. Metcalfe. "Pup: An
       Internetwork Architecture."  IEEE Transactions on Communications
       COM-28, 4, pp612-624, April 1980.

   2.  David D. Clark.  Names, Addresses, Ports, and Routes.  RFC-814,
       MIT-LCS, July 1982.

   3.  Yogan K. Dalal and Robert M. Metcalfe. "Reverse Path Forwarding
       of Broadcast Packets."  Comm. ACM 21, 12, pp1040-1048, December
       1978.

   4.  Ross Finlayson, Timothy Mann, Jeffrey Mogul, Marvin Theimer. A
       Reverse Address Resolution Protocol. RFC-903, Stanford
       University, June 1984.

   5.  R.M. Metcalfe and D.R. Boggs. "Ethernet: Distributed Packet
       Switching for Local Computer Networks."  Comm. ACM 19, 7,
       pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
       Center, reprinted in CSL-80-2.

   6.  Jeffrey Mogul. Broadcasting Internet Datagrams. RFC-919, Stanford
       University, October 1984.

   7.  David Plummer. An Ethernet Address Resolution Protocol. RFC-826,
       Symbolics, September 1982.

   8.  Jon Postel. Internet Protocol. RFC-791, USC-ISI, September 1981.

   9.  Jon Postel. Internet Control Message Protocol. RFC-792, USC-ISI,
       September 1981.

















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