X.500 is an OSI (Open System Information) based directory services protocol designed by the CCITT (International Telegraph and Telephone Consultative Committee).
X.500 provides distributed directory services to network users. The X.500 directory specifies a model for connecting directory services to form one distributed global directory. Each directory service holds a part of the global database and the directory information is made available via a server (called a Directory System Agent - DSA). The database is maintained locally. From the user point of view, the entire directory is accessible from the local server.
While most of the information available today via X.500 is about people and organizations, the design of the X.500 directory is also suitable for storing information about other entities (or objects), such as network resources, applications or hardware. Several projects are underway which utilise these directory capabilities (eg, the Internet RFCs (Request For Comments) are listed in the global directory).
Each item (entry) in the X.500 directory describes one object (eg, a person, a network resource, an organization) and has a Distinguished Name - DN (a unique identifier). It consists of a collection of attributes (eg, last name, organization name, e-mail,...- for a person). The information held in the X.500 directory (or Directory Information Base - DIB) is arranged hierarchically. This organization is called the Directory Information Tree (DIT). At the top-level is the root entry (the World), then the country level, then the organization level, and, eventually, the people, the resources, etc., at the bottom-level of the hierarchy.