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Let's start with the basics: what the heck is LDAP anyhoo?
The acronym LDAP stands for Lightweight Directory Access Protocol, which, according to the official specification at http://www.ietf.org/rfc/rfc2251.txt, is a protocol "designed to provide access to the X.500 Directory while not incurring the resource requirements of the Directory Access Protocol (DAP) [...] specifically targeted at simple management applications and browser applications that provide simple read/write interactive access to the X.500 Directory, and is intended to be a complement to the DAP itself".
Yup, it didn't make sense to me either.
Before you can understand LDAP, you need to first understand what a "directory service" is. A directory service is exactly what it sounds like - a publicly available database of structured information. The most common example of a directory service is your local Yellow Pages - it contains names, addresses and contact numbers of different businesses, structured by business category, all indexed in a manner that is easily browseable or searchable.
Like ice-cream, directory services come in many flavours. They may be local to a specific organization (the corporate phone book) or more global in scope (a countrywide Yellow Pages). They can contain different types of information, ranging from employee names, phone numbers and email addresses to domain names and their corresponding IP addresses They can exist in different forms and at different locations, either as a single electronic database within an organization's internal network or as a series of inter-connected databases existing at different geographical locations on a corporate extranet or the global Internet. Despite these differences, however, they all share certain common attributes: structured information, powerful browsing and search capabilities, and - in the case of distributed directories - inter-cooperation between the different pieces of the database.
Now, obviously, organizing information neatly in a directory is only part of the puzzle - in order for it to be useful, you need a way to get it out. If you're using the local phone book, getting information out it as simple as flipping to the index, locating the category of interest, and opening it to the appropriate page. If you're using an electronic, globally distributed directory service, however, you need something a little more sophisticated.
That's where LDAP comes in.
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