X.500 Standard status
(Implementors' Guide)

X.509 Related activities

How to be involved

More Information

Tutorial section 1
X.500 General

Tutorial section 2
X.509 specific

Other PKI organizations

edit SideBar


A little history

In 1984, ITU-T (then called CCITT) decided to develop a general-purpose directory specification. The immediate requirement was to provide a directory for Message Handling (X.400). ISO/IEC JTC1 started a similar activity. The two organizations agreed to merge the two activities into a single collaborative activity to avoid producing two different standards for the same purpose. This collaboration on what is called the OSI Directory was productive and it is still working.

As a result of that collaborative work ISO/IEC and ITU-T have published a virtually identical standard differing only in the opening foreword of the document. It is available as ISO/IEC International Standard 9594 (multiple parts) and as the ITU-T X.500 Series of Recommendations. A table listing the corresponding ISO and ITU-T identifiers can be found here. Since the X.500 identifier is easier to remember, the public has adopted it as the conventional reference to all the parts of the International Standard and all the Recommendations in the Series.

The OSI Directory work was started within the Open Systems Interworking (OSI) standardization activity. The OSI standards had little market penetration, mostly due to the industry acceptance of the TCP/IP and related specifications developed within the Internet environment. However, the OSI Directory gained general acceptance. The terms X.500 and the X.500 Directory Specifications are mostly used in preference to the more official title.

The X.500 Directory Specifications are documented as identical text within the ISO/IEC 9594 multi-part standard and in the ITU-T X.500 Series of Recommendations.

Five editions of the X.500 Directory Specifications have been issued so far:

First edition:

This is the first edition and is issued as the multi-part standard ISO/IEC 9594:1990 and as the CCITT X.500 (1988) Series of Recommendations. This edition specifies services, protocols and procedures necessary for basic directory operations. It specifies information models for how information is structured and it specifies some commonly usable information objects. In addition, it provides a common framework for general authentication techniques.

Second edition:

This second edition is issued as ISO/IEC 9594:1995 and as ITU-T X.500 (1993). This edition has added some very useful functions, like shadowing of directory information, access control and has significantly expanded the information model and administrative capabilities.

Third edition:

This third edition is issued as ISO/IEC 9594:1998 and as ITU-T X.500 (1997). It provides several minor and some major extensions. It adds a feature called contexts, which allows information to be distinguished according to the contexts in which it is being accessed. Another addition is the provision of OSI Management of the Directory. It has also added and extended security features.

Fourth edition:

This fourth edition is issued as ISO/IEC 9594:2001 and as ITU-T X.500 (2001). This edition provides several major extensions. It adds service management features, mapping-based matching, families of entries, support for a pure TCP/IP stack, etc.

Fifth edition:

This fifth edition is issued as ISO/IEC 9594:2005 and as ITU-T X.500 (2005). This edition provides maximum alignment with LDAP. Dependency external OSI specifications have been substantially reduced.

ISO/IEC and ITU-T may indicate different years for the same edition due to different rules. The ITU-T indicates the year in which the work has been approved, and ISO/IEC indicates the year of the official publication.

Page Actions

Recent Changes

Group & Page

Back Links