I have several IBM ThinkPads with an RJ-11 port that can be used for dialup internet connections. Currently I have one ThinkPad connected to a dialup modem, that connects to the internet through a modern ThinkPad with Ethernet. What would be the recommended solution for connecting multiple computers with dialup, without using a dialup modem for every ThinkPad?

So, I would like to have a star topology with an RS-232 uplink to a modern internet connected device. What is the recommended equipment to buy?

  • Forget about that useless modem stuff and get an old PCMCIA/PC-Card Ethernet adapter for each of the old Thinkpads.
    – Janka
    Dec 30 '18 at 2:51
  • Yes, I already own a few of them, but for nostalgic reasons I would also like a dialup network with multiple computers.
    – Eloy
    Dec 30 '18 at 3:05
  • You will be vastly disappointed with the speed. Do not expect more than a few kilobytes per second. Dec 30 '18 at 13:01
  • I know, I already have one computer with a dial up connection to the internet, but I would like to have more computers connected with dial up without using multiple individual modems.
    – Eloy
    Jan 1 '19 at 15:24

Dialup Internet connections are point-to-point connections; every client system connecting using a modem requires a server-side modem to accept the call.

If you want to skip the modems and just use direct RS-232 links that is possible but you will still need a bank of RS-232 ports on the server instead of a bank of modems. You can use a bunch of individual USB serial port adapters for this, or you can get a multiport serial adapter on your gateway machine. PPP is the protocol commonly used nowadays for such connections, but SLIP used to be popular and may be an option for you as well.

  • I see, but how do ISPs handle this? They have to use a device with multiple RJ-11 ports which handle incoming connections and connect them to an Ethernet network. I would like to buy such a device, if this is how it works.
    – Eloy
    Dec 30 '18 at 23:17
  • It's too complicated to cover in a comment. Search for "modem bank" for more information. Here's a link to the manual for one of the many products that might have been used in this situation: ftp1.digi.com/support/documentation/90031300c.pdf
    – Ken Gober
    Dec 31 '18 at 1:01

What OS are you running?

If you can run Windows 98 SE or Windows 2000 or XP, you can use ICS (Internet Connection Sharing).

It lets you use a dial up (or other type) of connection to the Internet on one computer and share it over LAN with others, basically implementing a rudimentary DHCP server and NAT.


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    The other way around. Also you need something to answer the modem call and do the necessary stuff. Dec 30 '18 at 13:02
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    Unfortunately, this does seem to be the opposite (sharing client connections, not sharing server connections). But it is exactly what I was thinking of when I didn't quite understand the question. Dec 30 '18 at 15:03

That RJ-11 port is meant to connect to a POTS line. You'll either need to get a separate phone line for each computer, or you'll need to set up your own mini-PBX (maybe something like this: https://www.aliexpress.com/item/Min-PABX-PBX-with-SIM-Card-System-for-Home-and-Office/32515623794.html)

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