pushp@nic.cerf.net (Pushpendra Mohta) (11/27/90)
In article <1990Nov26.233848.20828@nmt.edu> rmilner@zia.aoc.nrao.edu (Ruth Milner) writes: >In article <315@srchtec.UUCP> mra@srchtec.uucp (Michael Almond) writes: >>[...] >>One other item: PSI says we will not have a constant IP #. It supposedly >>changes each time we establish a connection with them. However, our address >>will remain constant (probably searchtech.com). I'm hoping this will not cause >>too many problem with our local setup, but I guess we're going to find out! > >Say what????? > From what the original poster has described, I suspect PSI is using the same terminal server as CERFnet uses for its dial up Internet Access Service, DIAL n' CERF . ( from cisco Systems) >I fail to see how this can be the case. Granted they can change their tables >any time they want, *but* they have the following restrictions: > > a) they have to use an Internet number assigned to them/you by the NIC The address assigned to the calling party is one on the subnet on which the terminal server resides. The network address is indeed one assigned by SRI-NIC. > b) the rest of the Internet has to understand the routing (if it's > advertised, and if it isn't no-one can reach you) See above. The route to the subnet/net is advertized to the rest of the Internet . > c) your system has to know *before establishing the connection* what > its own Internet address is > Not true. Your system has to know its IP address before estabilishing a *IP* connection. However, for example on a cisco terminal server, you may use BOOTP over the dial up connection to determine your IP address. >Are you sure that they meant your "IP #" changes? If you are connecting >through SLIP, then your circuit number might change each time, depending >how many other connections are being made at the same time. But your own >actual Internet address should stay the same. > >Even if it *could* be done, it would be such an administrative nightmare >trying to figure out "who's 123.456.789.10 this morning?" that I can't think >why they *would* do it. > There are ways to log correspondences between port numbers, IP numbers and user names. You can then use unix utilities like "last" etc on this data to find out who is what this morning. Not too difficult. I can't answer for PSI but here is why CERFnet does this. Dial-Up services are primarily targeted at the occasional user. Where IP address spaces are limited, it makes sense to recycle addresses from a small pool. This is not a problem for outgoing FTP or Telnet connections but it does make SMTP mail difficult. However as has been pointed out earlier , there are ways to get over this problem (UUCP, POP etc ) (In any case if you cannot do without an IP address of your own, with DIAL n' CERF it is possible to dynamically assign a password protected legal IP address of your choice .) Remember, the above applies to the occasional user.If someone is using their Internet connection often they should consider a ( perhaps slow speed ) leased line connection where their line would be hard wired into a port on the terminal server with a fixed IP address with the " same subnet as terminal server " proviso mentioned earlier. However pricing issues come into play here. >I dunno, maybe the connections are volatile enough that they just assign an >IP address in sequence or something every time a connections comes up, but >it sounds pretty weird to me. >-- I think you have the "gateway server model" of SLIP in mind where each end of the link has two different addresses and you can have a IP network at the remote end whose reachability may be advertized over the serial link. The services described above use the " terminal server model " of SLIP (cisco jargon) where there can only be *one* host and *one* IP address at the remote end. This host then appears to operate as a host on the same subnet as the terminal server. If the terminal server is on an ethernet, the remote host appears to the rest of the world as also on the same ethernet. There are terminal servers available in the market which follow the "gateway server model" on the dial-up link. So most of the above decribed features have come about because of the choice of terminal server provided by a rather popular vendor. >Ruth Milner >Systems Manager NRAO/VLA Socorro NM --pushpendra ---------------------------------------------------------------------- Pushpendra Mohta Network Co-ordinator CERFNet 1-800-876-CERF