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X.500

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.



X.400

An OSI and ITU standard messaging protocol. It is an application layer protocol (layer 7 in the OSI model). X.400 has been defined to run over various network transports including Ethernet, X.25, TCP/IP and dial-up lines. See messaging protocol, XAPIA and CMC. Following is the format of an X.400 address:


  /c=  /admd=  /prmd=  /o=  /s=  /g=


  X.400
  Code     Purpose

 /c=      country

 /admd=   administrative management domain
           (public email service)

 /prmd=   private management domain
           (inhouse email)

 /o=      organization

 /s=      surname

 /g=      given name





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