    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

    Postel                                                          [Page 1]

                                                              September 1981
    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

    Postel                                                          [Page 2]

                                                              September 1981
    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

    Postel                                                          [Page 3]

                                                              September 1981
    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

    Postel                                                          [Page 4]

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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

    Postel                                                          [Page 5]

                                                              September 1981
    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

    Postel                                                          [Page 6]

                                                              September 1981
    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.

    Postel                                                          [Page 7]