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




 

                                                          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