An advanced, hierarchical network directory service that comes with Windows servers and used for managing permissions and user access to network resources. Introduced in Windows 2000, Active Directory is a domain-based network that is structured like the Internet's Domain Naming System (DNS). Using the LDAP directory access protocol, a company's workgroups (departments, sections, offices, etc.) are assigned domain names similar to Web addresses, and any LDAP-compliant Windows, Mac, Unix or Linux client can access them. 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 domain-based network, LDAP, forests and trees, Internet domain name, ADSI and network directory.