IP to X.121 address mapping for DDN
RFC 1236

Document Type RFC - Informational (June 1991; No errata)
Last updated 2013-03-02
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Network Working Group                                         L. Morales
Request for Comments: 1236                                      P. Hasse
                                                               June 1991

                  IP to X.121 Address Mapping for DDN

Status of this Memo

   This memo defines a standard way of converting IP addresses to CCITT
   X.121 addresses and is the recommended standard for use on the
   Internet, specifically for the Defense Data Network (DDN).  This memo
   provides information for the Internet community.  It does not specify
   an Internet standard.  Distribution of this memo is unlimited.

1.  Overview

   The Defense Communication Agency (DCA) has stated that "DDN specifies
   a standard for mapping Class A addresses to X.121 addresses."
   Additionally DCA has stated that Class B and C IP to X.121 address
   mapping "standards are the responsibility of the administration of
   the Class B or C network in question".  Therefore, there is NO
   defined single standard way of converting Class B and Class C IP
   addresses to X.121 addresses.

   This is an important issue because currently there is no way for
   administrators to define IP to X.121 address mapping.  Without a
   single standard, in a multi-vendor network environment, there is no
   assurance that devices using IP and DDN X.25 will communicate with
   each other.

   The IP to X.121 address mapping of Class B and Class C IP addresses
   shall be implemented as described below.  This translation method is
   a direct expansion of the algorithm described in the "MIL-STD:  X.25,
   DDN X.25 Host Interface Specification" [1].  The translation method
   described below is TOTALLY independent of IP subnetting and of any
   masking that may be used in support of IP subnetting.

2.  Background

   All Internet hosts are assigned a four octet (32 bit) address
   composed of a network field and a local address field also known as
   the REST field [2] (see Figure 1 thru 3).  Two basic forms of
   addresses are provided:  (1) Physical addresses, correspond to the
   node number and DCE port number of the node to which the DTE is
   connected.  (2) Logical addresses, are mapped transparently by DCE
   software into a corresponding physical network address.

Morales & Hasse                                                 [Page 1]
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 "", n=26 h=9, l=0,
   and i=122.

   The different classes of Internet addresses [3] are illustrated

   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                                                 [Page 2]
RFC 1236          IP to X.121 Address Mapping for DDN          June 1991

      |        n      |       n       |       h       |       i       |
      |1 0|           NETWORK         |          Local Address        |
                      14 Bits                 16 Bits (REST Field)
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