Amazon.com Product Description
The GameCube Broadband Adapter allows for high-speed connection through a DSL or cable modem and is purely designed for online gaming--it doesn't support regular Internet access or Web browsing. The adapter is easily installed, simply snapping into serial port 1 on the underside of the console. The Broadband Adapter can also theoretically be used for linking two or more GameCube systems together, allowing for network gaming without the need for online access.
To use the Broadband Adapter you'll need a 10 megabit (10Base-T) or 100 megabit (100Base-TX) network cable, a broadband connection device such as a cable modem or DSL adapter with an available Ethernet port, and a valid account with a Broadband Internet Service Provider. Please be aware that some service providers, eg. AOL, do not allow outside software to access their services without the use of a special login tool. Please contact your service provider if you're not sure whether or not they allow this.