Newbie question: back-ending IIS with LDAP?

A customer is moving their web site from Netscape Enterprise
server on Unix (actually, IRIX) to IIS4 on NT4.  (Hey, not my call.)

For http authentication, it was simple enough to use a
Netscape "authdb" to store and manage IDs and passwords, but
using and programatically managing NT users is new to me -
especially since I'm looking at about 10,000 users who'll
have password-only access to these web pages.  Add to this
the memory requirements discussed at
http://www.ntfaq.com/Articles/Index.cfm?ArticleID=13518
and I begin to get concerned.

I'd much rather have IIS work off of an LDAP database, but
I'm not seeing a way to do this?

-John

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