As more and more
games have
multiplayer modes that require you to log on to a central server game-companies have come up with a clever new way to use CD-keys.
The keys of the different
players on the
servers are checked against each other and if the same key is found multiple times, the players who have identical keys are
banned from the server. This is an effective way to stop people from using
pirated versions, since all the people who have the pirated version more or less use the one same key.
Of course the companies probably do much more than just check for
identical keys, eg. check if the keys are consistent with the keys they have given out with the game, check which keys are used most and so on.