pentium5,
You will not find a URL which "just shows you how to set up an ISP). You instead must search for understanding in each of the areas I listed earlier.
Even in your "tiny isp", you must know basically the same things as the big isps, because you are using the same technology, only on a smaller scale.
Also, you have the added problem of not having a block of valid IP addresses to dynamically assign for each modem, as users dial in. Thus, you need to have a "private" network, using addresses such as 192.168.xxx.xxx, or 10.xxx.xxx.xxx, which are a group of addresses which cannot not be seen externally on the world wide web. These addresses are intended for internal networks, where many computers can share an internet connection with the one computer (firewall) that has a valid external IP address.
Thus, you need to run your server as a firewall, doing Network Address Translation (NAT), so that all those users can surf the web using your server's IP address.
As I said, you are not going to find ONE place that lists everything you need in a nice, simple order. There are just too many possible variations of the ISP scenario to make a nice, neat list. So, you need to identify exactly what it is you want to do, and then learn each piece. There are plenty of URLs to show you how to do the following:
(try searching at www.google.com/linux)
- PPP
- NAT (search for "ipchains")
- DHCP
- DNS (you probably don't need to run your server as a DNS server, unless you want to host domains, but you still need to understand the basics of what it is, and how it works)
- Mail (specifically, sendmail, postfix, or qmail)
Look for forums and newsgroups also, such as comp.os.linux.networking. Spend some time at www.ugu.com and search through the HOWTO's at http://docs.linux.com/.
There are plenty of resources, but you won't get a "cookbook", with 1-2-3 instructions. You have to put the ideas together yourself.