When the University of Michigan developers designed Lightweight Directory Access Protocol (LDAP) in 1989, they wanted to free clients from the heavyweight Directory Access Protocol (DAP) for X.500 directory access. So they placed an LDAP server between the X.500 directory and the client. The LDAP server translated a directory request from an LDAP client on a TCP/IP network from the client's language to that used by X.500. The LDAP server then sent the request to an X.500 server.
LDAP reduced development work and resource requirements on the client side. However, it shifted development tasks to the LDAP server, because the LDAP server had to speak complicated X.500 protocols to talk to the X.500 directory. This shift led to LDAP's independence from X.500.
Today, a standalone LDAP server can supply a complete LDAP directory running on a TCP/IP network without the need of X.500. LDAP is no longer just a protocol, but a directory service in itself. (For more information about the evolution of LDAP, see Craig Zacker, "LDAP and the Future of Directory Services," Part 1 and Part 2, October and November 1997.