CnCNet

CnCNet is an open-source (client and server) package that allows online play through the LAN interface for Tiberian Dawn, Red Alert, Tiberian Sun, Red Alert 2 and their expansions. It's the defacto online service used by Tiberian Dawn and Red Alert players.

An application called 'cncnet.exe' is placed in the game folder and executed, it will start the game connected to CnCNet, the player then needs to open the LAN menu. The latest fan patches for Tiberian Dawn, Red Alert and Tiberian Sun contains a -LAN argument that can be used to make 'cncnet.exe' start the game with the LAN menu open.

Site: http://cncnet.org/