Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!mordor!sri-spam!ames!aurora!labrea!agate!ucbvax!ccq.bbn.COM!pogran
From: pogran@ccq.bbn.COM (Ken Pogran)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP class B and C to X.25 address translation
Message-ID: <8801270631.AA18694@ucbvax.Berkeley.EDU>
Date: 26 Jan 88 23:02:52 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 37

Barry,

How about the mapping that Andy Malis suggested in his message of
19 January?

     There is an informal standardization for Class B: the first two
     octets of the IP address are the network number, the third octet
     is treated identically to the second octet of Class A addresses,
     and the fourth octet is treated identically to the fourth octet
     of Class A addresses.  The third octet of Class A addresses is
     dropped completely in Class B addresses.
     
     There is absolutely no standardization for Class C, because there
     are so few local network address bits to play with.  The host
     network software support person for Class C nets must provide his
     or her own mapping between the Class C addresses and X.25
     addresses for that net.  For example, the five most significant
     bits of the fourth octet of the Class C address could be the host
     number, and the three least significant bits the PSN number.  It
     is a compromise between the number of PSNs on the network and the
     maximum number of hosts on a PSN.

We've just put in a small "campus-area" Class B network of C/30
PSNs using this mapping.

If my memory serves me correctly (and it turns out that I'm the
author of the text in the DDN X.25 spec that you quoted -- though
not the originator of the algorithm), an earlier draft of the DDN
X.25 spec contained both Class A and Class B mappings. In review,
it was pointed out to us that since this was a _DDN_ X.25 spec,
and DDN was a Class A network, there was no need for a Class B
mapping in the DDN-specific document.  In retrospect, it would
have been a good idea to put out the Class B mapping as an RFC at
the same time the DDN spec was published!  

I think it's now time to put out that RFC ...

Ken Pogran