Lightweight Directory Access Protocol (LDAP) is a protocol for accessing on-line directory services.
The IETF designed and specified LDAP as a better way to make use of X.500 directories - having found the original Directory Access Protocol (DAP) too complex for simple internet clients to use. LDAP defines a relatively simple protocol for updating and searching directories running over TCP/IP.
The common term "LDAP directory" can mislead. No specific type of directory is an "LDAP directory". One could reasonably use the term to describe any directory accessible using the LDAP protocol and which can identify objects in the directory with X.500 identifiers. Directories such as OpenLDAP and its predecessors from the University of Michigan, though primarily designed as native repositories optimized for access by LDAP rather than as a gateway to X.500 protocols as was provided in ISODE, are nevertheless no more "LDAP directories" than any other directory accessible by the LDAP protocol.
LDAP has gained wide support from vendors such as:
Apple (through Open Directory)
Microsoft (through Active Directory)
Oracle (through Oracle Internet Directory)
as well as in open source/free software implementations such as OpenLDAP.
An LDAP directory entry consists of a collection of attributes with a name, called a distinguished name (DN), which refers to the entry unambiguously. Each of the entry's attributes has a type and one or more values. The types are typically mnemonic strings, like "cn" for common name, or "mail" for e-mail address. The values depend on the type, and most non-binary values in LDAPv3 use UTF-8 string syntax . For example, a mail attribute might contain the value "firstname.lastname@example.org". A jpegPhoto attribute would contain a photograph in binary JPEG/JFIF format.
LDAP directory entries feature a hierarchical structure that reflects political, geographic, and/or organizational boundaries. In the original X.500 model, entries representing countries appear at the top of the tree; below them come entries representing states or national organizations. Typical LDAP deployments use DNS names for structuring the top levels of the hierarchy. Further below might appear entries representing people, organizational units, printers, documents, or just about anything else.