Address mappings
RFC 796

Document Type RFC - Historic (September 1981; No errata)
Last updated 2014-05-06
Stream Legacy
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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
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