The Lightweight Directory Access Protocol (LDAP) is a set of open protocols used to access centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource-intensive. For this reason, LDAP is sometimes referred to as "X.500 Lite." The X.500 standard is a directory that contains hierarchical and categorized information, which could include information such as names, addresses, and phone numbers.

Like X.500, LDAP organizes information in a hierarchal manner using directories. These directories can store a variety of information and can even be used in a manner similar to the Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled network.

In many cases, LDAP is used as a virtual phone directory, allowing users to easily access contact information for other users. But LDAP is more flexible than a traditional phone directory, as it is capable of referring a querent to other LDAP servers throughout the world, providing an ad-hoc global repository of information. Currently, however, LDAP is more commonly used within individual organizations, like universities, government departments, and private companies.

LDAP is a client/server system. The server can use a variety of databases to store a directory, each optimized for quick and copious read operations. When an LDAP client application connects to an LDAP server, it can either query a directory or attempt to modify it. In the event of a query, the server either answers the query locally, or it can refer the querent to an LDAP server which does have the answer. If the client application is attempting to modify information within an LDAP directory, the server verifies that the user has permission to make the change and then adds or updates the information.

This chapter refers to the configuration and use of OpenLDAP 2.0, an open source implementation of the LDAPv2 and LDAPv3 protocols.