LDAP - Lightweight Directory Access Protocol What is LDAP ? LDAP , Lightweight Directory Access Protocol , is an Internet protocol that email and other programs use to look up information from a server. Every email program has a personal address book, but how do you look up an address for someone who's never sent you email? How can an organization keep one centralized up-to-date phone book that everybody has access to? That question led software companies such as Microsoft, IBM, Lotus , and Netscape to support a standard called LDAP. "LDAP-aware" client programs can ask LDAP servers to look up entries in a wide variety of ways. LDAP servers index all the data in their entries, and "filters" may be used to select just the person or group you want, and return just the information you want. For example, here's an LDAP search translated into plain English: "Search for all people located in Chicago whose name contains "Fred" that have an emai...