Re: DDN X.25 mapping

Andy Malis (malis@CC5.BBN.COM)
Mon, 14 Sep 87 10:47:48 -0400


Ron is correct, RFC 796 only discusses the Class A mapping
between IP addresses and ARPANET/MILNET addresses. He described
the generally accepted mapping for Class B IP addresses, using
the third octet for the host number and the fourth octet for the
PSN (IMP) number.

PLEASE remember to follow the algorithm in section A-5 of the DDN
X.25 Host Interface Spec (BBN Report 5476, in the DDN Protocol
Handbook, Vol 1, as section 6.6) for deriving the X.25 address,
so that logical addressing will work correctly. All you have to
do is substitute the third octet of a Class B IP address for the
2nd octet of a Class A IP address as discussed in the algorithm.
This is on pages A-9 and A-10 of the spec (pages 1-497 and 1-498
of the Protocol Handbook).

There is no generally accepted standard for Class C addresses,
nor am I aware of any Class C PSN-based networks. Certainly, 4
bits for each of the host and PSN numbers would work - it depends
on the number of PSNs in the network and the maximum number of
hosts on each PSN. This mapping should probably be customized on
a network-by-network basis. For consistency with the Class A and
Class B mappings, the host number bits should be in the more
significant bits of the 4th octet.

Andy Malis
PSN Development

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