Arpa address 3486

Anne & Lynn Wheeler

If a node is an IMP, then 100 nodes is not inconsistent with 250 hosts. Remember the mapping of IP addresses on net 10 (and 26 and 21 and ...)

10.a.b.c - 10 was the network number, i.e. Arpanet - c was the IMP number - a was the port number on that IMP - b was usually zero, but some devices (early routers) (sometimes mentioned as multiplexers) used this to put several hosts behind a box attached to one IMP port. Most IMPs had at least two hosts: A mainframe (PDP-10 clbutt time- sharing system, or IBM-360 clbutt mainframe, typically) and a TAC - Terminal Access Controller (later we would call this a terminal server) which was the same kind of mini as the IMP but with TTY port cards, typically with modems hanging off them.

By 1982, there were many different kinds of host attachments some of which were capable of extended distances. ACC in Santa Barbara where I was working was on a VDH (very distant host) port off the IMP in El Segundo (i.e. Los Angeles Air Force Base).

LH (Local Host) - the fastest port type, but limited to about 50 feet, IIRC DH (Distant Host) - same circuit (BBN-1822) but clocked a bit slower with pulse shaping to avoid ringing and overshoots on cables up to a couple hundred feet. VDH (Very Distant Host) - an HDLC-derivative protocol to go with 9600 bps sync modems (or 56 kbps DDS if you could afford that). ECU (Error Control Units) could convert a DH circuit to an HDLC framed MIL-STD-188 synchronous serial port with automatic retransmission to correct bit errors on the long-haul link.

Manufacturing (and maintaining) ECUs was ACC's bread and butter. When ARPAnet split to create MILNET, ACC landed on the MILNET side, even though we were firmly civilian. The routers that connected net 10 and net 26 were called "mail bridges". This simple trick of nomenclature ensured that bureaucrats never understood that they were protocol agnostic and transparent. The only time I remember them being deliberately turned off was during the Morris worm attack.

Lars Poulsen