RFC 1236 (rfc1236) - Page 2 of 7

IP to X

RFC 1236          IP to X.121 Address Mapping for DDN          June 1991


   To provide flexibility, Internet addresses are divided into 3 primary
   classes:  Class A, Class B, and Class C.  These classes allow for a
   large number of small and medium sized networks.  The network
   addresses used within the Internet in Class A, B, and C networks are
   divided between Research, Defense, Government, (Non-Defense) and
   Commercial uses.

   As described in the MIL-STD:  X25, an IP address consists of the
   ASCII text string representation of four decimal numbers separated by
   periods, corresponding to the four octets of a thirty-two bit
   Internet address.  The four decimal numbers are referred to in this
   memo as network (n), host (h), logical address (l), and Interface
   Message Processor (IMP) or Packet Switch Node (PSN) (i).  Thus, an
   Internet address maybe represented as "n.h.l.i" (Class A), "n.n.h.i"
   (Class B), or "n.n.n.hi" (Class C), depending on the Internet address
   class.  Each of these four numbers will have either one, two, or
   three decimal digits and will never have a value greater than 255.
   For example, in the Class A IP address "26.9.0.122", n=26 h=9, l=0,
   and i=122.

   The different classes of Internet addresses [3] are illustrated
   below:

   Class A:

   The highest-order bit is set to 0.
   7-bits define the network number.
   24-bits define the local address.
   This allows  up to 126 class A networks.
   Networks 0 and 127 are reserved.


      |       n       |       h       |       l       |       i       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|   NETWORK   |                 Local Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             7 Bits                  24 Bits (REST Field)

                                   Figure 1

   Class B:

   The two highest-order bits are set to 1-0.
   14-bits define the network number.
   16-bits define the local address.
   This allows up to 16,384 class B networks.





Morales & Hasse