LDAP (Lightweight Directory Access Protocol) is a protocol for accessing online directory services. It is most commonly used to provide online access to e-mail address directories, and most e-mail client programs support LDAP address books. It's also commonly used as an authentication directory.

LDAP is based on TCP, and servers will typically listen on well known ports 389 (for plain text) or 636 (over SSL). The protocol can be used to access a standalone LDAP directory service or a X.500 directory server. The University of Michigan produced the original LDAP server "slapd", which has evolved into a product called OpenLDAP. The Red Hat Directory Server (formerly Netscape Directory Server), iPlanet, Novell's NDS, and Microsoft's Active Directory all support LDAP to varying degrees.

Unlike most other internet protocols, LDAP uses a binary encoding rather than a simple text stream. LDAP requests and responses are encoded with a scheme called BER, for Basic Encoding Rules.

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