X.500 Standard status
X.509 Related activities
How to be involved
Tutorial section 1
Tutorial section 2
X.509 at work
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:
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.