Windows NT 4 authentication

Windows NT is a secure operating system in that users have to provide a user ID and password in order to gain access to the system. When a user logs on, the information is compared with a database of user information that is maintained on the server. If the user ID and password are valid, restrictions such as logon times and workstation restrictions are compared. If there are no restrictions in force, a token is generated by the system and passed back to the authentication system, which then allows the user access.

The token created for the user contains information such as the user's ID (which is expressed as a number called a security identifier [SID] rather than the user's name) and any group memberships that the user holds. Each time a user attempts to access a protected resource, the token is compared against the access control list for that resource. If you add a user to a group after the user has logged on, the user must log off and log back on again in order for the change to take effect.