IEN 91






                            Address Mappings






                           Jonathan B. Postel














                               2 May 1979




                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291

                             (213) 822-1511


                                                               J. Postel
IEN 91                                                               ISI
                                                              2 May 1979



                            ADDRESS MAPPINGS
                            ----------------



Internet Address
----------------

   This memo describes  the relationship  between address fields used in
   the Internet Protocol (IN) [1] and several specific networks.

   An internet  address  is a 32 bit quantity,  divided  into an  8  bit
   network number and a 24 bit local address as shown below.

      +--------+--------+--------+--------+
      !    Net !      Local  Address      !
      +--------+--------+--------+--------+

   The local address  carries  information  to address  a  host  in  the
   network  identified  by the network number.  Since each network has a
   particular address format and length, the following section describes
   the mapping  between  internet local addresses and the actual address
   format used in the particular network.

Internet to Local Net Address Mappings
--------------------------------------

   The following  transformations are used to convert internet addresses
   to local net addresses and vice versa:

      ARPANET
      -------

         The ARPANET (with 96 bit leaders) has 24 bit addresses.  The 24
         bits are assigned  to host,  logical  host,  and IMP fields  as
         illustrated  below.   These 24 bit addresses  are used directly
         for the 24 bit local address of the internet address.

         The network number of the ARPANET is 10.

         +--------+--------+--------+
         !  HOST  !   LH   !  IMP   !
         +--------+--------+--------+
              8        8        8





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                                                              2 May 1979
IEN 91                                                  Address Mappings



      SATNET
      ------

         The Atlantic  Satellite Packet Network has 16 bit addresses for
         hosts.  These addresses may be assigned independent of location
         (i.e.,  ground station).  It is also possible to assign several
         addresses  to one physical  host,  so the addresses are logical
         addresses.   The 16 bit SATNET address is located in the 24 bit
         internet local address as shown below.

         The network number of the SATNET is 4.

         +--------+----------------+
         !  ZERO  !        HOST    !
         +--------+----------------+
              8           16

      PRNET
      -----

         The Packet  Radio networks  use 16 bit  addresses.   These  are
         independent  of location (indeed the hosts may be mobile).  The
         16 bit PRNET addresses are located in the 24 bit internet local
         address as shown below.

         The network numbers of the PRNETs are:

            BBN-PR      1
            SF-PR-1     2
            SILL-PR     5
            SF-PR-2     6
            BRAGG-PR    9
            DC-PR      20

         +--------+----------------+
         !  ZERO  !        HOST    !
         +--------+----------------+
              8           16

      LCSNET
      ------

         The LCS NET at MIT's Laboratory  for Computer  Science  uses 32
         bit addresses  of several  formats.   Please  see [2] for  more
         details.   The most common format locates the low order 24 bits
         of the 32 bit LCS NET address  in the  24  bit  internet  local
         address, as shown below.

         The network number of the LCS NET is 18.



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                                                              2 May 1979
IEN 91                                                  Address Mappings



         +--------+--------+--------+
         !   SUB  !RESERVED!  HOST  !
         !   NET  !        !        !
         +--------+--------+--------+
              8        8        8

      EDN
      ---

         The Experimental  Data Network  at  the  Defense  Communication
         Engineering  Center (DCEC) uses the old 1822 32 bit leader, and
         so has 8 bit addresses.   These 8 bit EDN addresses are located
         in the 24 bit internet local address as shown below.

         The network number of the EDN is 21.

         +------+--+----------+------+
         !      ! H!          !      !
         ! ZERO ! O!   ZERO   !  IMP !
         !      ! S!          !      !
         !      ! T!          !      !
         +------+--+----------+------+
             6    2     10        6

References
----------

   [1]     Jonathan B. Postel, "Internetwork Datagram Protocol - Version
           4," Information Sciences Institute, IEN 80, February 1979.

   [2]     Noel Chiappa,  David Clark,  David  Reed,  "LCS  Net  Address
           Format,"  M.I.T.  Laboratory  for  Computer  Science  Network
           Implementation, Note No.5, IEN 82, February 1979.



















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