How can I set username and password for my wireless users for connecting to the Internet?

[496]

I have a wireless network which everyone can connect to use the organization's portal. But I want to restrict users from connecting to the Internet. I have seen some organization do this by setting username and passwords for legal users. e.g. when users type a website address, another page opens and asks for username and password. How can I do this? Is it a DSL modem configuration?
( I use a TP-Link wireless ADSL router to manage users and connect the organization to the outside.)

[Fred]

You can not. you would need to pass ALL Internet traffic through a server set up to do the routing. Then issue them all user names on that server. but your connection is totally stupid if you do not set encryption. EVERY packet anyone passes across the wireless is readable by ANYONE nearby with a wireless machine. That can give away EVERY piece of private data in your company.