Broadband Adapter

The Nintendo GameCube Broadband Adapter (BBA) and the Nintendo GameCube Modem Adapter are a network adapter and 56k dial-up modem, respectively, for the Nintendo GameCube. They were produced by Conexant and made in the Philippines. The adapters fit flush into "Serial Port 1" on the underside of the GameCube and add an Ethernet port or telephone port to the side of the console, allowing the console to communicate with other consoles through a network.

As of, Dolphin's main branch has BBA emulation support. However, it requires a Virtual Private Network (VPN) in order to function. BBA emulation is not supported on OSX.

For instructions on how to set up a VPN and BBA emulation, use the links below.
 * Google Code Dolphin Wiki: BBA: How to use the Broadband Adapter
 * Dolphin Forums: BBA working :)