An international standard (1988) developed jointly by the International Organization for Standardization (ISO) and the ITU-T to support the e-mail name and address lookup requirements of X.400. X.500 provides for global directory services that theoretically enable network managers to store information about all users, machines, and applications in a distributed fashion. X.500 is a very robust global directory standard that requires significant computational resources to implement and, therefore, is criticized for being over-engineered. See also e-mail, ISO, ITU-T, LDAP, X.400, and X Series.
An OSI protocol for managing online directories of users and resources. X.500 can be used to support X.400 and other messaging systems, but it is not restricted to e-mail usage. It provides a hierarchical structure that fits the world's classification system: countries, states, cities, streets, houses, families, etc. The goal is a global directory. The DIB, DSA and DCA An X.500 directory is called a "Directory Information Base" (DIB) or white pages. The program that maintains the DIBs is called a "Directory Server Agent" (DSA). A Directory Client Agent (DCA) is used to search DSA sites for names and addresses. Replication and Access Control Published in 1988, the 1993 edition of X.500 added replication and access control. Using the Directory Information Shadowing Protocol (DISP), replication allows a portion of the Directory Information Tree (DIT) to be copied between nodes. Access control allows or denies access to a particular attribute of a directory entry based on the identity of the requesting user. See X.400.