Yes and no. Earlier boxes would interoperate, _if addresses on each end were chosen properly_. Modern handling of addresses on hosts (for the 'is this destination on my physical network' step of the packet-sending algorithm) did not come in until RFC-1122 (October 1989); prior to that, lots of host code probably tried to figure out if the destination was class A, B or C, etc, etc. Also, until RFC-826 (ARP, November 1982) pretty much all the network interfaces (and thus the code to turn the 'destination IP address' into an attached physical network address, for the first hop) were things like ARPANet that no longer exist, so you could't _actually_ fire up one of them unless you do something like the 'ARPANet emulation' that the various PDP-10 simulators use to allow old OS's running on them to talk to the current Internet. What are they, if not TCP/IP? Not the modern variant, of course, but then again, nothing before the early 90's is truly 'modern TCP/IP'. I read this, BITD, but don't recall much about it. I was not impressed by the coding style. I don't know. IIRC, that compiler supported both styles. It had to have been a later compiler than the one that came with V6, that didn't support longs. But I don't recall any bug with long support in the typetter C compiler we had at MIT. No. That clearly belongs to the DTI one. (The difference between V3 and V4, while significant, aren't enough to make the DTI not 'TCP/IP in C for Unix'.) If you want to say 'first V4TCP/IP in C for Unix', maybe; I'd have to look for dates on the one done at MIT for V6, that may be earlier, but I don't think so. (Check the minutes in the IEN's, that's probably the best source of data on progress of the various implementations.) Here's my _guess_ - ask someone from Bell for a sure answer. You're using 20/20 hindsight. At that point in time, it was not at all obvious that TCP/IP was going to take over the world. There were a couple of alternatives for moving data around that Bell used - Datakit, and UUCP - and they worked pretty well, and there was no reason to pick up on this TCP/IP thing. I suspect that it wasn't until LAN's became popular than TCP/IP looked like a good thing to have - it fits very well with the capabilities most LANs had (in term of the service provided to things attached to them). Datakit was its own thing, and for UUCP you'd have to provide a reliable stream, and TCP/IP 'just did that'. Noel