

Network Working Group                                          J. Postel
Request for Comments:  796                                           ISI
Replaces: IEN 115                                         September 1981
                            ADDRESS MAPPINGS
                            ----------------


Internet Addresses
------------------

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

   An internet address is a 32 bit quantity, with several codings as
   shown below.

   The first type (or class a) of address has a 7-bit network number and
   a 24-bit local address.

                           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 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   NETWORK   |                Local Address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Class A Address

   The second type (or class b) of address has a 14-bit network number
   and a 16-bit local address.

                           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 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 0|           NETWORK         |          Local Address        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Class B Address

   The third type (or class c) of address has a 21-bit network number
   and a 8-bit local address.

                           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 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 1 0|                    NETWORK              | Local Address |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Class C Address

   The local address carries information to address a host in the
   network identified by the network number.  Since each network has a



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RFC 796                                                 Address Mappings



   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:

      AUTODIN II
      ----------

         The AUTODIN II has 16 bit subscriber addresses which identify
         either a host or a terminal.  These addresses may be assigned
         independent of location.  The 16 bit AUTODIN II address is
         located in the 24 bit internet local address as shown below.

         The network number of the AUTODIN II is 26 (Class A).

         +----------------+
         |  HOST/TERMINAL |   AUTODIN II
         +----------------+
                 16

         +--------+--------+--------+--------+
         |   26   |  ZERO  |  HOST/TERMINAL  |   IP
         +--------+--------+--------+--------+
              8        8           16





















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RFC 796                                                 Address Mappings



      ARPANET
      -------

         The ARPANET (with 96 bit leaders) has 24 bit addresses.  The 24
         bits are assigned to host, logical host, and IMP leader fields
         as illustrated below.  These 24 bit addresses are used directly
         for the 24 bit local address of the internet address.  However,
         the ARPANET IMPs do not yet support this form of logical
         addressing so the logical host field is set to zero in the
         leader.

         The network number of the ARPANET is 10 (Class A).

         +--------+--------+--------+
         |  HOST  |  ZERO  |  IMP   |   ARPANET
         +--------+--------+--------+
              8        8        8

         +--------+--------+--------+--------+
         |   10   |  HOST  |   LH   |  IMP   |   IP
         +--------+--------+--------+--------+
              8        8        8        8

      DCNs
      ----

         The Distributed Computing Networks (DCNs) at COMSAT and UCL use
         16 bit addresses divided into an 8 bit host identifier (HID),
         and an 8 bit process identifier (PID).  The format locates
         these 16 bits in the low order 16 bits of the 24 bit internet
         address, as shown below.

         The network number of the COMSAT-DCN is 29 (Class A), and of
         the UCL-DCN is 30 (Class A).

         +--------+--------+
         |   HID  |   PID  |   DCN
         +--------+--------+
              8        8

         +--------+--------+--------+--------+
         |   18   |  ZERO  |   HID  |   PID  |   IP
         +--------+--------+--------+--------+
              8        8        8        8






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RFC 796                                                 Address Mappings



      EDN
      ---

         The Experimental Data Network at the Defense Communication
         Engineering Center (DCEC) uses the same type of addresses as
         the ARPANET (with 96 bit leaders) and has 24 bit addresses.
         The 24 bits are assigned to host, logical host, and IMP leader
         fields as illustrated below.  These 24 bit addresses are used
         directly for the 24 bit local address of the internet address.
         However, the IMPs do not yet support this form of logical
         addressing so the logical host field is set to zero in the
         leader.

         The network number of the EDN is 21 (Class A).

         +--------+--------+--------+
         |  HOST  |  ZERO  |  IMP   |   EDN
         +--------+--------+--------+
              8        8        8

         +--------+--------+--------+--------+
         |   21   |  HOST  |   LH   |  IMP   |   IP
         +--------+--------+--------+--------+
              8        8        8        8

      LCSNET
      ------

         The LCS NET at MIT's Laboratory for Computer Science uses 32
         bit addresses of several formats.  Please see [3] 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 (Class A).

         +--------+--------+--------+
         | SUBNET |RESERVED|  HOST  |   LCSNET
         +--------+--------+--------+
              8        8        8

         +--------+--------+--------+--------+
         |   18   | SUBNET |RESERVED|  HOST  |   IP
         +--------+--------+--------+--------+
              8        8        8        8





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RFC 796                                                 Address Mappings



      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 (Class A)
            SF-PR-1     2 (Class A)
            SILL-PR     5 (Class A)
            SF-PR-2     6 (Class A)
            BRAGG-PR    9 (Class A)
            DC-PR      20 (Class A)

         +--------+--------+
         |       HOST      |   PRNET
         +--------+--------+
                 16

         +--------+--------+--------+--------+
         |   net  |  ZERO  |       HOST      |   IP
         +--------+--------+--------+--------+
              8        8           16
























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RFC 796                                                 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 (Class A).

         +--------+--------+
         |       HOST      |   SATNET
         +--------+--------+
                 16

         +--------+--------+--------+--------+
         |    4   |  ZERO  |       HOST      |   IP
         +--------+--------+--------+--------+
              8        8           16

      WBCNET
      ------

         The Wideband Communication Satellite Packet Network (WBCNET)
         Host Access Protocol (HAP) has 16 bit addresses for hosts.  It
         is possible to assign several addresses to one physical host,
         so the addresses are logical addresses.  The 16 bit WBCNET
         address is divided into a HAP Number field and a Local Address
         field, and is located in the 24 bit internet local address as
         shown below.  Please see [2] for more details.

         The network number of the WBCNET is 28 (Class A).

         +--------+--------+
         | HAP NUM| LCL ADD|   WBCNET
         +--------+--------+
              8        8

         +--------+--------+--------+--------+
         |   28   | HAP NUM|  ZERO  | LCL ADD|   IP
         +--------+--------+--------+--------+
              8        8        8        8






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RFC 796                                                 Address Mappings



References
----------

   [1]     Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
           Protocol Specification," RFC 791, USC/Information Sciences
           Institute, September 1981.

   [2]     Pershing J., "Addressing Revisited," Bolt Beranek and Newman
           Inc., W Note 27, May 1981.

   [3]     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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