The protocols defined by X.500 include:
Because these protocols used the OSI networking stack, a number of alternatives to DAP were developed to allow Internet clients to access to the X.500 Directory using the TCP/IP networking stack. The most well-known alternative to DAP is Lightweight Directory Access Protocol (LDAP). While DAP and the other X.500 protocols can now use the TCP/IP networking stack, LDAP remains a popular directory access protocol.
The primary concept of X.500 is that there is a single Directory Information Tree (DIT), a hierarchical organization of entries which is distributed across one or more servers. An entry consists of a set of attributes, each attribute with one or more values. Each entry has a unique Distinguished name, formed by combining its Relative distinguished name (RDN), one or more attributes of the entry itself, and the RDNs of each of the superior entries up to the root of the DIT. As LDAP implements a very similar data model to that of X.500, there is further description of the data model in the article on LDAP.
X.520 and X.521 together provide a definition of a set of attributes and object classes to be used for representing people and organizations as entries in the DIT is one of the most widely deployed white pages schema.
X.509, the portion of the standard providing for an authentication framework, is now also widely used outside of the X.500 directory protocols. It specifies a standard format for public-key certificates.
|ITU-T number||ISO/IEC number||Title of Standard|
|X.500||ISO/IEC 9594-1||The Directory: Overview of concepts, models and services|
|X.501||ISO/IEC 9594-2||The Directory: Models|
|X.509||ISO/IEC 9594-8||The Directory: Public-key and attribute certificate frameworks|
|X.511||ISO/IEC 9594-3||The Directory: Abstract service definition|
|X.518||ISO/IEC 9594-4||The Directory: Procedures for distributed operation|
|X.519||ISO/IEC 9594-5||The Directory: Protocol specifications|
|X.520||ISO/IEC 9594-6||The Directory: Selected attribute types|
|X.521||ISO/IEC 9594-7||The Directory: Selected object classes|
|X.525||ISO/IEC 9594-9||The Directory: Replication|
|X.530||ISO/IEC 9594-10||The Directory: Use of systems management for administration of the Directory|
The authors of RFC 2693 (concerning SPKI) note that "The original X.500 plan is unlikely ever to come to fruition. Collections of directory entries... are considered valuable or even confidential by those owning the lists and are not likely to be released to the world in the form of an X.500 directory sub-tree." and that "The X.500 idea of a distinguished name (a single, globally unique name that everyone could use when referring to an entity) is also not likely to occur."