Nintendo GameCube Broadband Adapter
From the Super Mario Wiki
The Nintendo GameCube Broadband Adapter is an attachment for the Nintendo GameCube game system, allowing to the console to connect via LAN with another Nintendo GameCube console. A few games can employ this connection for multiplayer purposes, such as Mario Kart: Double Dash!! To connect, owners of this device must register their network cards and configure the systems for the DHCP server. Some games can support up to eight consoles connected if each system has its GameCube discs.