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rfc:ien:ien91
 
 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

Postel [Page 1]

                                                            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.

Postel [Page 2]

                                                            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.

Postel [Page 3]

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