A recommendation from the International Telecommunication Union (ITU) that specifies a global, hierarchical directory service. Features of X.500 include the following:
X.500 defines a global directory service that consists of several components. From an administrative point of view, the building blocks of the X.500 directory service are Directory Management Domains (DMDs). An X.500 DMD is a collection of X.500 components that includes at least one Directory System Agent (DSA) and is managed by a Domain Management Organization (DMO). There are two types of DMDs:
Three main components are involved in maintaining and accessing X.500 directory services:
Graphic X-3. X.500. The X.500 directory service.
To access information in the directory, a DUA connects to a local DSA and queries the directory by using the Directory Access Protocol (DAP), the standard X.500 protocol for locating, accessing, and modifying information in an X.500 directory. Various attribute-based search methods are possible using X.500-based directory services, including the following:
When a DUA issues a query, the query travels through a chain of DSAs and a result set travels back along the same chain. These queries use DAP, while DSAs communicate with each other using the Directory System Protocol (DSP).
X.500 forms the basis of Active Directory in Windows 2000, the directory service of Microsoft Exchange Server, and Novell Directory Services (NDS).
A simplified version of DAP called the Lightweight Directory Access Protocol (LDAP) is more widely implemented than the feature-heavy DAP. LDAP was developed by the University of Michigan for use on TCP/IP networks such as the Internet and is widely implemented in Simple Mail Transfer Protocol (SMTP) client software such as Microsoft Outlook Express for querying online directories about SMTP users.