Re: IP class B and C to X.25 address translation

Andy Malis (malis@CC5.BBN.COM)
Tue, 19 Jan 88 13:53:06 -0500

The following message didn't seem to make it out of my host when
I sent it the first time. Since this is a topic that comes up
from time to time, I thought I would resend it.


------- Forwarded Message

To: "Barry D. Hassler" <wrtfac!hassler@LOGNET2.ARPA>
cc: malis,
Subject: Re: IP class B and C to X.25 address translation
In-reply-to: Your message of Sun, 27 Dec 00 19:87:13 +0000.
Date: Mon, 28 Dec 87 11:44:33 -0500
From: Andy Malis <>

Barry asked the following:

        Has there been any standardization on the translation of
        Class B or C IP addresses to X.25 addresses? I am aware
        of the translation standard for Class A addresses, but
        have not seen any for B or C.


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.


------- End of Forwarded Message

This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:40:41 GMT