As soon as Internet email became popular, it was clear we needed a good phone book. Printed directories were obsolete before the ink was dry. Older Internet methods of looking up names, such as whois, Ph, or finger, were limited or arcane. 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's why software companies such as Microsoft, IBM, Lotus, and Netscape agreed 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 email address. Please return their full name, email, title, and description." (However, many email clients have more limited search and retrieval options.)
"Permissions" are set by the administrator to allow only certain people to access the LDAP database, and optionally keep certain data private. LDAP servers also provide "authentication" service, so that web, email, and file-sharing servers (for example) can use a single list of authorized users and passwords.
LDAP was designed at the University of Michigan to adapt a complex enterprise directory system (called X.500) to the modern Internet. A directory server runs on a host computer on the Internet, and various client programs that understand the protocol can log into the server and look up entries. X.500 is too complex to support on desktops and over the Internet, so LDAP was created to provide this service "for the rest of us."
LDAP servers exist at three levels: There are big public servers such as BigFoot and Infospace, large organizational servers at universities and corporations, and smaller LDAP servers for workgroups.
You probably already have an LDAP-aware client installed on your computer. Most modern email clients are set up to search an LDAP directory for email addresses. These include Outlook, OS X Mail, Eudora, Netscape, QuickMail Pro, and Mulberry.
LDAP has broader applications, such as looking up services and devices on the Internet (and intranets). Netscape Communicator can store user preferences and bookmarks on an LDAP server. There is even a plan for linking all LDAP servers into a worldwide hierarchy, all searchable from your client.
LDAP promises to save users and administrators time and frustration, making it easy for everyone to connect with people without frustrating searches for email addresses and other trivia.
