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 email 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