For the most part, if you installed your Linux system with the cable modem connected, this is probably already working, and you have nothing left to do. If, however, you are already up and running and you are just now getting a cable modem, it is probably time for a few pointers. Quite frankly, these days (with a modern Linux distribution), there isn't much to it.
To begin with, cable modems aren't modems in the classic sense. The so-called modem is connected to your cable TV service on one side and to an Ethernet card inside your PC on the other. High-speed access through your phone company's DSL service is similar, in that they will provide you with an external, modemlike device (in many cases, it is really a router) that also connects to an Ethernet card.
The Ethernet card (which should be automatically detected by your system) gets an IP address from the cable modem via the Dynamic Host Configuration Protocol (DHCP). Although this address may appear permanent, in that it rarely (if ever) changes, it is nevertheless dynamic, because your actual Ethernet card gets its address whenever it connects.
The process of getting your system configured varies a little bit from distribution to distribution, but only cosmetically. When you install your new Ethernet card (for access through the cable modem), it will be autodetected by the system on reboot. As part of that process, the system will ask whether you want to configure the card. The answer is yes, of course. Next, the system will ask whether you want to supply an IP address or have it autoconfigure via DHCP. With a cable modem, as with DSL, autoconfiguring is what you want.
Now that I've shown you how incredibly easy it is to do this, I'm going to mention that there are many different providers of high-speed cable and DSL access. What this means is that if your system doesn't autorecognize and configure your connection, you may need to do one of these things. For cable modems, the answers vary, but start by checking out the Cable Modem HOWTO at http://www.tldp.org/HOWTO/Cable-Modem/index.html for details on your particular geographic location.
If you are on a phone company DSL service, look on your distribution disks for the rp-pppoe package (PPP over Ethernet) and install it (many distributions will have it installed by default since DSL is so popular). You can also get the package from Roaring Penguin at http://www.roaringpenguin.com/pppoe/, but you probably have it on your CDs. Make sure you check there first.
Once the package is installed, open a shell (Konsole) and switch to the root user. Do this by typing "su - root" at the shell prompt. You'll be asked for the root password. One you have entered it, type this command:
This is basically a fill-in-the-blanks session. Your phone company will have assigned you a username and password, along with some connection information, and will have provided you with this. Answer all of the questions (the information is case sensitive, so be careful entering it). When you have answered everything, type the following at your shell prompt:
That's it. You have no doubt guessed that there is also an adsl-stop command as well as adsl-status (which, among other things, will tell you your IP address). If you install the RPM package from your distribution, adsl-start will run automatically when you reboot your system, so you don't need to worry about it each time.