A CD key comes with each copy of Neverwinter Nights and each expansion pack. This 35-character alphanumeric code is used for authentication, both during installation of the game and when using the multiplayer aspect of the game. The code for the base game is also used to validate premium modules. The key is entered during the game's installation and stored in the nwncdkey.ini file (if the key is accepted by the installer). When starting or joining a multiplayer game, the game's CD keys are authenticated by BioWare's master server (if the master server is available). Furthermore, each computer connecting to the game must have its own set of CD keys.
The master server has been offline since the summer of 2011, so key verification has not occurred since then. This prevents logging in to servers that require master server authentication (a setting in the server's nwnplayer.ini file). Otherwise, the situation is similar to what has always been the case for LAN games with no internet connection available: the authentication step is simply skipped. It can, though, take some time for the game to determine that the master server is not available if an internet connection is available.
The CD key identifies the computer on which Neverwinter Nights is installed. A player may have multiple CD keys (from purchasing the game multiple times, for use on multiple computers), and a single CD key may be used by multiple players (who are sharing a single computer). A single CD key may not, however, be used on multiple computers. (In theory a single computer could have multiple CD keys, but the game has an internal check that prevents more than one instance of the game from running at any given time, so this question is academic.)
As an exception to the "one computer" rule, copies of the game purchased from GoG.com all come with the same CD key (a consequence of GoG.com removing copy protection from all games they sell). This causes no problems in single-player, but can be a problem in multiplayer. For those wishing to use Neverwinter Nights for multiplayer, GoG.com will issue unique CD keys to their customers who request them.[1]
Public CD key[]
CD keys are private, not to be shared with others. However, there is a shortened form of the key, called the public CD key, which is transmitted to the host of a multiplayer game and which can be obtained with the NWScript command GetPCPublicCDKey(). This shortened form is an 8-character code composed of alternating characters in the CD key, starting with the second and ending with the sixteenth. For example:
CD-key: 12345 - 67890 - ABCDE - FGHIJ - KLMNO - PQRST - UVWXY Public key: 2 4 6 8 0 B D F