AD (Active Directory)
Active Directory (AD) is Microsoft’s enterprise directory and identity platform, first released with Windows 2000 and now the dominant identity provider in enterprise environments worldwide. It combines four technologies into a single integrated system: LDAP as the directory access protocol for querying and modifying identity data; Kerberos 5 as the authentication protocol for issuing tickets that prove identity without transmitting passwords; DNS as the service location mechanism that clients use to discover domain controllers, Kerberos KDCs, and LDAP servers; and Group Policy as the configuration management system that pushes security settings, software installation, and policy enforcement to every joined machine. These four components are inseparable in practice: a Linux host joining an AD domain receives a Kerberos principal in AD’s KDC, a machine account object in the AD LDAP directory, a DNS record for its hostname, and (optionally) Group Policy Objects applied to it. The AD forest is the trust boundary: multiple domains can exist within a forest and all share a common schema, configuration, and Global Catalog, with transitive Kerberos trust between them.