Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!bloom-beacon!gatech!mcnc!decvax!ucbvax!nap1.arpa!hassler
From: hassler@nap1.arpa (Barry D. Hassler)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP class B and C to X.25 address translation
Message-ID: <12620.8801261955@nap1.arpa>
Date: 25 Jan 88 18:27:47 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 134



Its been some time since I sent out my original message  concern-
ing  the subject, and I have received a few responses to my ques-
tion, but noone answered the question I asked (maybe I  asked  it
poorly).

My original question concerned the translation from IP  addresses
to  X.25  addresses for X.25 links, such as those on MILNET (each
answer I received told me about subnetting,  which  I  do  under-
stand).   The X.25 standard used by DDN contains an appendix out-
lining the address translation, but it only  works  properly  for
Class   A  addresses.  The  algorithm  itself  can  be  found  in
NETINFO:X25.DOC.1 on sri-nic.arpa.

The algorithm used is as follows (copied out of X25.DOC, appendix
A):

------------------------------------------------------
                              A-8
A-5 Derivation of DDN  X.25 Addresses

        All DDN hosts are assigned addresses by the Administration.
  The address of a DDN host may be obtained from the Network
  Information Center (NIC), represented as an ASCII text string in
  what  is  called "host table format".  This section describes the
  process by which DDN X.25 addresses in the format described in
  Section 2.1.1 may be derived from addresses in NIC host table
  format.

        A NIC host table address consists of the ASCII text string
  representations 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
  section as "n", "h", "l", and "i."  Thus, a host table address
  may be represented as "n.h.l.i"  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 host table address
  "10.2.0.124", n=10, h=2, l=0, and i=124.  To convert a host table
  address to a DDN X.25 address:

        1.  If h < 64, the host table address corresponds to
            the DDN X.25 physical address

                           ZZZZ F IIIHHZZ (SS)

            where:

                 ZZZZ = 0000
                         as required in Section 2.1.1.1.1;

                 F = 0   because the address is a physical
                         address;

                 III     is a three decimal digit
                         representation of "i", right-adjusted
                         and padded with leading zeros if
                         required;

                 HH      is a two decimal digit representation
                         of "h", right-adjusted and padded
                         with leading zeros if required;,

                 ZZ = 00
                         and

                 (SS)    is optional, as described in Section
                         2.1.1.1.4.




                              A-9
          In the example given above, the host table address
          10.2.0.124 corresponds to the DDN X.25 physical
          address 000001240200.

      2.  If h > 64 or  h  =  64,   the  host   table  address
          corresponds to the DDN X.25 logical address

                      ZZZZ F RRRRRZZ (SS)

          where:

               ZZZZ = 0000
                      as required in Section 2.1.1.1.1;

               F = 1  because the address is a logical
                      address;

               RRRRR  is a five decimal digit
                      representation of the result "r" of
                      the calculation

                                 r = h * 256 + i

                      (note that the decimal representation
                      of "r" will always require five
                      digits);

               ZZ = 00
                      and

               (SS)   is optional, as described in Section
                      2.1.1.1.4.

          Thus,    the host    table   address    10.83.0.207
          corresponds   to  the  DDN  X.25  logical  address
          000012145500.

     In both cases, the "n" and "l"  fields  of  the  host   table
address are not used.

-------------------------------------------------------------
Note that while this algorithm can be used on class B and  C  ad-
dresses,  it  maps  out  to  duplicate X.25 addresses for certain
Class B and C addresses, since it only  takes  into  account  the
second and fourth octets of the address.

Are there any other standards being  used  for  this  translation
that  will provide a one-to-one mapping of both class B and C ad-
dresses to X.25 addresses?  I am running a quickly  growing  net-
work  of  hosts,  and have receiveed several requests already for
X.25 connections to our gateway.  I am very  concered  about  the
address translations for our class B network addresses (subnetted
of course). I use portions of the third octet (depending upon the
size  of  the subnet, and whether or not it needs to be subnetted
further) for the subnetwork number and find that that octet  does
not  enter in the X.25 address at all. Therefore, If I would have
two X.25 hosts behind my gateway with  addresses  129.48.1.1  and
129.48.32.1,   they  would  both  have  the  same  X.25  address:
000011228900.

-BDH