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 Enterprise Linux and Fedora Core 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 costing less than $50 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. I recently purchased a very satisfactory gateway router having four Ethernet ports and a built-in wireless adapter for $30, net of rebates.
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:
This somewhat dated HOWTO examines the technology behind DSL and offers advice on subscription, installation, configuration, and troubleshooting for Linux users.
This HOWTO relates to an obsolete version of Red Hat Linux, but nevertheless contains much useful information.
These resources are both a bit out of date, but are the most current available. That they haven't been recently updated seems to demonstrate that most Linux users are using gateway routers rather than Linux hosts as their means of connecting to their cable or DSL provider.