How can I restrict and control access to my server?

All major servers have features that allow you to limit access to particular sites, and many clients have authentication features that allow you to identify specific users. There is a tutorial on security and user authentication with the NCSA server and Mosaic available, written by Marc Andreessen (URL is http://wintermute.ncsa.uiuc.edu:8080/auth-tutorial/tutorial.html ). See your server documentation for further information.
World Wide Web FAQ