LDAP's history is quite sordid. The story begins with the OSI (Open Systems Interface) networking system, a cradle-to-grave network protocol suite that was misguidedly adopted as an international standard in the mid-1980s. The OSI system as a whole proved to be a big flop, but several of its component protocols (LDAP among them) have enjoyed a macabre afterlife in mutant forms adapted to life in the TCP/IP world.
LDAP was originally designed as a simple gateway protocol that would allow TCP/IP clients to talk to the X.500 directory servers that ran on OSI systems. Over time, it became apparent both that X.500 was going to die out and that UNIX really needed a standard directory of some sort. These factors have led to LDAP being developed as a full-fledged directory system in its own right (and perhaps to its no longer being quite so deserving of the "L", i.e., it's not quite so "Lightweight". Although LDAP is certainly lightweight in comparison to X.500).
The book has a neat diagram on page 6 comparing X.500 over OSI to LDAP over TCP/IP.
By the late 1990s, the vast majority of LDAP implimentations were LDAP version 2 (henceforth, LDAPv2). LDAPv2 lacked many of the features needed for LDAP to reach the same level of functionality of other directory services (e.g. DNS), and LDAS was widely regarded as limitted and oft useless. However, in 1997 there had been an ehancement to the protocol (LDAPv3) which provided a number of very important improvements:
Strong Authentication via SASL
Integrity and Confidentiality Protection via TLS (SSL)
Internationalization through the use of Unicode
Referrals and Continuations
Schema Discovery
Extensibility (controls, extended operations, and more)
LDAPv2 is now considered historical. As deploying both LDAPv2 and LDAPv3 simultaneously can be quite problematic, LDAPv2 should be avoided. LDAPv2 is disabled by default with most LDAP servers (including OpenLDAP).
