X.500 Directory Service is a standard way to develop an electronic directory of people in an organization so that it can be part of a global directory available to anyone in the world with Internet access. Such a directory is sometimes called a global White Pages directory. The idea is to be able to look up people in a user-friendly way by name, department, or organization. Many enterprises and institutions have created an X.500 directory. Because these directories are organized as part of a single global directory, you can search for hundreds of thousands of people from a single place on the World Wide Web.
The X.500 directory is organized under a common "root" directory in a "tree" hierarchy of: country,organization, organizational unit, and person. An entry at each of these levels must have certain attributes; some can have optional ones established locally. Each organization can implement a directory in its own way as long as it adheres to the basic schema or plan. The distributed global directory works through a registration process and one or more central places that manage many directories.
Providing an X.500 directory allows an organization to make itself and selected members known on the Internet. Two of the largest directory service providers are InterNIC, the organization that supervises domain name registration in the U.S., and ESnet, which maintains X.500 data for all the U.S. national laboratories. ESNet and similar providers also provide access to looking up names in the global directory, using a number of different user interfaces including designated Web sites, whois, and finger. These organizations also provide assistance to organizations that are creating their own Directory Information Tree (DIT).
In X.500, each local directory is called a Directory System Agent (DSA). A DSA can represent one organization or a group of organizations. The DSAs are interconnected from the Directory Information Tree (DIT). The user interface program for access to one or more DSAs is a Directory User Agent (DUA). DUAs include whois, finger, and programs that offer a graphical user interface. X.500 is implemented as part of the Distributed Computing Environment (DCE) in its Global Directory Service (GDS). The University of Michigan is one of a number of universities that use X.500 as a way to route e-mail as well as to provide name lookup, using the Lightweight Directory Access Protocol (LDAP).