X500

X.500 is a family of standards for directory services developed by the International Telecommunication Union (ITU) and the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC). It defines a distributed directory service that provides information about entities such as people, organizations, and devices within a networked environment. The core idea is to offer a scalable, hierarchical repository of attribute-based data that can be queried and updated across multiple servers.

The architecture centers on a Directory Information Tree (DIT), in which entries are organized hierarchically and identified by distinguished names (DNs). Each entry contains a set of attributes and belongs to one or more object classes that specify permitted attributes and behavior. The directory information base is stored on Directory System Agents (DSAs), while Directory User Agents (DUAs) provide client access. The protocol stack is OSI-based, with the Directory Access Protocol (DAP) used by DUAs to read and modify directory data and the Directory System Protocol (DSP) used for communications between DSAs.

Key concepts include schema definitions for object classes and attribute types, the notion of a DIT for scalable organization, and the use of unique identifiers for entries and attributes. X.500 also specifies naming, security, and access control mechanisms to govern directory operations. It influenced later directory services, most notably LDAP (Lightweight Directory Access Protocol), which preserves the X.500 data model and semantics while adopting a simpler, TCP/IP-based protocol. Today, X.500 is less commonly deployed in new systems, but its concepts remain foundational in modern directory infrastructures and academic discussions of directory semantics.