An advanced, hierarchical directory service that comes with Windows servers. Introduced with Windows 2000 servers, Active Directory uses the LDAP directory access protocol and is built upon the Internet's Domain Naming System (DNS). Workgroups are given domain names, just like Web sites, and any LDAP-compliant Windows, Mac, Unix or Linux client can access it.

Active Directory can function in a heterogeneous, enterprise network and encompass other directories including NDS and NIS+. Cisco supports Active Directory in its IOS router operating system. See forests and trees, Internet domain name, ADSI and directory service.