"Max Eskin" <kurtkilgor(a)bigfoot.com> wrote: There used to be (probably still are) X.25 public data networks. "Public" means "not private" -- effectively, the network owner sells connections and bandwidth to paying external customers, not just to users within the owner's company. You as a customer would pay your money, and the network owner would deal with details like telco so that you effectively get RS-232 synchronous serial port(s) that want to see and hand out X.25 packets at your various locations, and X.25 network addresses corresponding to those ports. Or maybe you would pay for the telco stuff and an access charge to the network provider, and get a port on the network provider's X.25 packet switch. This might be of use to you if you had a need for computer/computer or computer/terminal communication between distant locations. Host computers (minicomputers and larger) often had X.25 network interfaces available as an option, and software that supported computer-to-computer communications and/or terminal-to-computer communications over it. Terminals didn't, but you could get devices called PADs (Port Access Devices? I can't remember) that had connections for terminals, an X.25 connection for the network, and could either act something like a multiplexer, running all the terminal sessions over a permanent virtual circuit (PVC) to a host's address, or like a terminal server, using switched virtual circuits (SVCs) to connect to any host address on the network. Or you might just have a host and a casual need for connectivity from terminals in remote locations, e.g. you've got some database and you sell access to it. So rather than installing a modem rack, you get a connection to Telenet and tell your customers that they need to dial in to their local Telenet access number (which is really a Telenet-owned modem pool with something-like-PADs behind it) with a terminal and modem and connect to such-and-such address. Either way, the X.25 public data network solution could be cheaper than installing point-to-point telco data lines between your hosts and terminals, and the X.25 network provider could offer internal redundant routing that might be prohibitively expensive for you to implement on your own. Now, that said, X.25 networks do not need to be public. If you have the wherewithal, you can build your very own X.25 network out of the same sorts of hardware that the public data network providers use. I guess the Iraqi government could have one or several. And I remember a data center that, in the late 1980s, got fed up with its computer-to-computer communications being done entirely of point-to-point links between computers (with a protocol that wasn't smart enough to forward messages -- if you wanted to go from host A to host C, and the only connection was through B, you had to log on to B between the computers. X.25 can also be used to transport other protocols' datagrams, e.g. IP. In fact the US Defense Data Network was IP-over-X.25 and may still be for all I know. I think this sort of thing is either incompatible with or distinct from using X.25 to carry host/terminal traffic (in much the same way that e.g. rlogin and telnet are distinct -- they both run over TCP but use different protocols, and different ports so TCP can multiplex them), so you can't just dial into a PAD and telnet to a host doing IP over X.25, unless the PAD also does telnet and IP over X.25. -Frank McConnell