Class C nets and DDN X.25


art@acc.arpa
15 Sep 87 09:26:00 PST


>For the record, both BBN and PANDA TOPS-20 assume that any Class C
>PSN-based network will have the host number in the high order 2 bits,
>and the PSN in the low order 6 bits of the fourth octet. This is the
>format which we used in the good old days of NCP and 32-bit leaders,
>for those of us old enough to remember that far back (am I betraying
>my age????).

This was fine for the original IMPS which only had four host ports.
(I remember the ruggedized Honeywell 516 based IMP at UCSB which was
one of the four nodes in the first incarnation of the ARPANET)
But newer IMPS have many more host ports. Our in-house test IMP has
20 host ports, so we would probably recommend at least 5 bits for host
number. This leaves room for a net of about 6 IMPS (reserve 0 and 7).
I don't know of anyone really running a class C IMP net. We have a
class C number assigned for our test IMP but haven't put it to use
yet.

AHIP-E could affect all this, but it may never be deployed.

                                        Art Berggreen
                                        art@acc.arpa

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