Programming a license server

Hi all,
I would like to implement a license server which will distribute a bunch of licenses to multiple instances of an application.
Example 1000 user login licenses will be distributed to the applications running on the network. Applications will get user licenses automatically through the network connection to License Server. When one application goes down, then all licenses used by that application should be released and made available for others.
Does anybody have any idea? Any API already available for that?
Regards,
Sertac