rfc950.txt

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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         Source address:          36.40.0.62
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

      Note that the gateway uses the narrowest possible broadcast to
      reply.  Even so, the over use of broadcasts presents an
      unnecessary load to all hosts on the subnet, and so the use of the
      "anonymous" (0.0.0.0) source address must 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 Mask Request = AM1
         Code:                    0
         Mask:                    0

      36.40.0.62 should respond exactly as in the previous case.

         Source address:          36.40.0.62
         Destination address:     36.40.0.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.0.0

   2.  A Class B Network Case

      For this case, assume that the requesting host is on class B
      network 128.99.0.0, has address 128.99.4.123, that there is a
      gateway at 128.99.4.62, and that a 6-bit wide subnet field is in
      use, that is, the address mask is 255.255.252.0.

      The host sends the ICMP request to 255.255.255.255:

         Source address:          128.99.4.123
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0


Mogul & Postel                                                 [Page 13]



RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


      The gateway can then respond directly to the requesting host.

         Source address:          128.99.4.62
         Destination address:     128.99.4.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

      In the diskless workstation case the host sends:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

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

         Source address:          128.99.4.62
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0

      If broadcasting is not allowed 128.99.4.123 sends:

         Source address:          128.99.4.123
         Destination address:     128.99.4.62
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      128.99.4.62 should respond exactly as in the previous case.

         Source address:          128.99.4.62
         Destination address:     128.99.4.123
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.252.0




Mogul & Postel                                                 [Page 14]



RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


   3.  A Class C Network Case (illustrating non-contiguous subnet bits)

      For this case, assume that the requesting host is on class C
      network 192.1.127.0, has address 192.1.127.19, that there is a
      gateway at 192.1.127.50, and that on network an 3-bit subnet field
      is in use (01011000), that is, the address mask is 255.255.255.88.

      The host sends the ICMP request to 255.255.255.255:

         Source address:          192.1.127.19
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      The gateway can then respond directly to the requesting host.

         Source address:          192.1.127.50
         Destination address:     192.1.127.19
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88.

      In the diskless workstation case the host sends:

         Source address:          0.0.0.0
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

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

         Source address:          192.1.127.50
         Destination address:     255.255.255.255
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88.

      If broadcasting is not allowed 192.1.127.19 sends:




Mogul & Postel                                                 [Page 15]



RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


         Source address:          192.1.127.19
         Destination address:     192.1.127.50
         Protocol:                ICMP = 1
         Type:                    Address Mask Request = AM1
         Code:                    0
         Mask:                    0

      192.1.127.50 should respond exactly as in the previous case.

         Source address:          192.1.127.50
         Destination address:     192.1.127.19
         Protocol:                ICMP = 1
         Type:                    Address Mask Reply = AM2
         Code:                    0
         Mask:                    255.255.255.88

Appendix III.  Glossary

   Bridge

      A node connected to two or more administratively indistinguishable
      but physically distinct subnets, that automatically forwards
      datagrams when necessary, but whose existence is not known to
      other hosts.  Also called a "software repeater".

   Gateway

      A node connected to two or more administratively distinct networks
      and/or subnets, to which hosts send datagrams to be forwarded.

   Host Field

      The bit field in an Internet address used for denoting a specific
      host.

   Internet

      The collection of connected networks using the IP protocol.

   Local Address

      The rest field of the Internet address (as defined in [3]).

   Network

      A single Internet network (which may or may not be divided into
      subnets).


Mogul & Postel                                                 [Page 16]



RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


   Network Number

      The network field of the Internet address.

   Subnet

      One or more physical networks forming a subset of an Internet
      network.  A subnet is explicitly identified in the Internet
      address.

   Subnet Field

      The bit field in an Internet address denoting the subnet number.
      The bits making up this field are not necessarily contiguous in
      the address.

   Subnet Number

      A number identifying a subnet within a network.

Appendix IV.  Assigned Numbers

   The following assignments are made for protocol parameters used in
   the support of subnets.  The only assignments needed are for the
   Internet Control Message Protocol (ICMP) [5].

   ICMP Message Types

      AM1 = 17

      AM2 = 18


















Mogul & Postel                                                 [Page 17]



RFC 950                                                      August 1985
Internet Standard Subnetting Procedure


References

   [1]  Mogul, J., "Internet Subnets", RFC-917, Stanford University,
        October 1984.

   [2]  Postel, J., "Multi-LAN Address Resolution", RFC-925,
        USC/Information Sciences Institute, October 1984.

   [3]  Postel, J., "Internet Protocol", RFC-791, USC/Information
        Sciences Institute, September 1981.

   [4]  Finlayson, R., T. Mann, J. Mogul, M. Theimer, "A Reverse Address
        Resolution Protocol", RFC-903, Stanford University, June 1984.

   [5]  Postel, J., "Internet Control Message Protocol", RFC-792,
        USC/Information Sciences Institute, September 1981.

   [6]  Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
        University, October 1984.

   [7]  GADS, "Towards an Internet Standard Scheme for Subnetting",
        RFC-940, Network Information Center, SRI International,
        April 1985.

   [8]  Croft, B., and J. Gilmore, "BOOTP -- UDP Bootstrap Protocol",
        RFC-951, Stanford University, August 1985.

   [9]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC-943,
        USC/Information Sciences Institute, April 1985.

   
















 

Mogul & Postel                                                 [Page 18]