X.500
The ITU-T series of directory standards defining a hierarchical, distributed directory of named entries, the model on which LDAP and X.509 certificates are built.
X.500 is a series of ITU-T recommendations, published jointly with ISO/IEC as the 9594 series and first issued in 1988 by its predecessor body, the CCITT, that defines a distributed directory service. Entries sit in a hierarchical tree, the Directory Information Tree, and each is identified by a distinguished name built from its path down the tree, for example country, organisation, organisational unit and common name. The series also defines the Directory Access Protocol (DAP), through which clients query the directory.
Full X.500 with DAP ran over the OSI protocol stack and was considered heavy to deploy, which led to the Lightweight Directory Access Protocol, first designed as a simpler route to X.500 directories over TCP/IP and now the common directory protocol in enterprise networks. LDAP kept X.500’s naming model, so LDAP distinguished names follow the same structure. X.509, the certificate format used across public key infrastructure, is also part of the series: it began as the directory’s authentication framework, which is why certificate subject and issuer names are X.500 distinguished names.
Exam relevance: a scenario is likely to ask which standard LDAP descends from, or what a distinguished name identifies. Candidates are expected to trace the line from X.500 to LDAP and X.509, and to recognise the directory’s hierarchical naming.