SATZ@MATHOM.CISCO.COM (Greg Satz) (09/13/87)
What should a DDN X.25 interface do about mapping to X.121 addresses when connected to a class B or C network? I remember seeing something about this before somewhere. If I remember correctly the C and D octets should be used for the imp and host octets for class B addresses. For class C, the upper four bits and the lower four bits of octet D should be used for the imp and host octets. Is this correct? -------
ron@TOPAZ.RUTGERS.EDU.UUCP (09/14/87)
There is no standard for the IMP to Class B and C addresses that I know of. HOWEVER, both BRL and the Canadian version of MILNET use the C octet for host number and the D octet for imp number (essentially sliding the HOST byte right one byte). -Ron
malis@CC5.BBN.COM (Andy Malis) (09/14/87)
Greg, 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. Regards, Andy Malis PSN Development
MRC@PANDA.COM (Mark Crispin) (09/14/87)
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????). DEC TOPS-20 doesn't handle non-class A PSN's at all, nor does any version of TOPS-20 I know of handle X.25 interfaces (the DEC IMPterface is an 1822 device). I think that DEC is adopting the PANDA algorithm though. I know of no controversy about the format a Class B PSN network address, or, at least TOPS-20 and Unix agree on having the host number in the third octet and the PSN number in the fourth octet. -------