User Principal Name in Microsoft Windows 2000, is the logon name of a user on the network. The user principal name consists of the associated name of the User object in Active Directory that is associated with the user, followed by an ampersand (&), and then typically followed by the Domain Name System (DNS) name of the container in Active Directory in which the User object resides. The user principal name must be unique for every User object in Active Directory.
If the User object in Active Directory for user Jeff Smith has the name jsmith and if this User object resides in the container whose DNS name is sales.microsoft.com, the default form of Jeff Smith’s user principal name is as follows:
jsmith@sales.microsoft.com
An alternative form of the user principal name for Jeff Smith is
jsmith@microsoft.com
In other words, the suffix for the user principal name of a user can be either the name of the tree or the name of the domain that resides above the object named in the tree. Note the similarity between the user principal name for Jeff Smith and what you might expect his Simple Mail Transfer Protocol (SMTP) e-mail address to be. The difference is that the user principal name is merely a property that is assigned to the user and can be set to any value.
The user principal name is one of three types of names by which objects in Active Directory can be known. The others are the distinguished name and the relative distinguished name.