RFC 1236 (rfc1236) - Page 1 of 7

IP to X

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Network Working Group                                         L. Morales
Request for Comments: 1236                                      P. Hasse
                                                                 USAISEC
                                                               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
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