At one time, setting up a cable or DSL modem for use with Linux was difficult. The new Network Administration Tool included in Red Hat Linux 8.0 often makes it a snap. But sometimes, the Tool fails to successfully configure the cable or DSL modem.
In such a case I recommend using a cable/DSL gateway router. Netgear and Linksys, among others, manufacture popular models. These inexpensive devices?often less than $100 retail?sit between your cable or DSL modem and your home network. Cable/DSL gateway routers generally provide a masquerading firewall and DNS proxy services. Better models have multiple ports so that you can connect several PCs without buying additional hubs or switches. Some recent models even provide a wireless LAN.
Because such devices are designed to work with as many cable and DSL configurations as possible, they work right out of the box most of the time. It's true that they generally provide no function that couldn't be provided?at least in principle?by a Linux PC. But they consume less power, occupy less space, make less noise, and require less configuration and administration than a Linux PC. I retired a quite venerable Pentium 166 Linux PC from cable modem gateway duty several years ago and have never regretted the decision.
Some useful resources when setting up Linux to access a cable or DSL modem include: