Paul Koning wrote: Johnny, the original DECnet manuals showed pictures of area routers that were interconnected by WAN links. Each site had its own area number. In fact the name "area routing" implies clearly that the concept was meant to set up DECnet networks that were geographically separated. I don't remember those pictures, but this is an issue that generated a lot of debate and a lot of annoyance in the DECnet architecture group. The pictures, atleast in the RSX manuals, are very simplistic. Like five nodes, maybe four areas, and spread between the west and east coast of the US. ... One reason for separating systems in different areas was that rebooting the pc's would generate so many DECnet state up and down messages that PDP- 11's and the older VAX systems choked in their console output. That's a good one. (Actually, the way to deal with that is to turn the logging to console off for that event...) Yes, that's a valid example. The purpose of area routing is to avoid having things get too large. There were various opinions on how big an area (or Phase III network) could reasonably be. Originally the limit was assumed to be about 32, without any basis that I know of. That too produced a pile of debate, and later on it became obvious you could go much higher. But clearly you couldn't run a flat Phase III style network with 10,000 nodes. Well, phase III networks could never be more than 255 nodes. :-) For that reason area routing was introduced, so each instance of the routing algorithm (in-area and level 2) would deal with a reasonably small topology. Again debates broke out whether a full area (1000 nodes) was acceptable. It turned out yes, but certainly keeping it smaller, say to 100-200, would be a good thing if possible. When phase IV came, and the node address was extended to 16 bits, it's obvious (I think) that you didn't want a flat 16-bit address space. So then it becomes a question of how to split that address up. By that time, I think they had come to the conclusion that a single network could definitely be more than 255 nodes. 1023 seemed like a good size, leaving 63 areas. (Neither node 0, nor area 0 is permitted.) Geography isn't a consideration unless the links are slow enough to make route change propagation an issue. Corporate organization (different departments) CERTAINLY isnt' a consideration, though some people tried to argue that it was valid to do so. Well, level 1 routers gives you pretty much the same isolation as areas. Unless you consider the relative size of the level 1 routing messages a problematic burden compared to level 2 routing messages. However, I'm not even sure you can make the system not transmit level 1 routing messages on all interfaces, meaning they will go out anyway, even if you only have another area at the other end of the line. Johnny