I am planning to buy 64 bit windows enterprise server 2008 for web server. It comes with 25 user licenses. I am planning to deploy a web application into that server and web application uses windows authontication. Number of users who access this web application would be 100+. Now my question is that Do I require additional user licenses apart from 25 users comes with the Windows server, in order to make this application accessiable for 100+ users?
If you use normal Server Standard, Enterprise, or Datacenter licensing; and the application uses any form of login (Windows Integrated, AD, whatever) you need a CAL for every user. Also if you use SQL server you need SQL CALs for every user as well.
Web Edition however does not have this licensing requirement. Web Edition can only be used for certain thing, but running a public website is what it's made for. (There are of course exceptions to this general rule; most revolve around the idea that you can't use Web Edition to get out of buying CALs for other products that you use through the Web Edition server).
There are other licensing options out there too, but most of them are meant for larger deployments than 100 users.
That basically is it. You decided to jump into the ship and get an expensive license that is pretty much useless because you did not care to do your homework first.