What is CD key for cs 1.6 zombie mode?

A CD key for Counter-Strike 1.6 (CS 1.6) is a unique alphanumeric code used to authenticate and activate the game. In the context of zombie mode, which is a popular custom mod for CS 1.6, players typically need a valid CD key to connect to servers hosting this mode. However, CD keys are not specifically tied to game modes like zombie mode; they are required for the base game itself. Always ensure you obtain CD keys through legitimate means to support the developers and avoid potential issues.