MS IIS Access Control question

[Edward Spodick]

Perhaps one of you can help me with a problem.  We have an MS IIS v5 web server.
Goal - global access to the server, no restrictions (anonymous)
- add access control limits for 1 directory, such that on-campus users (by IP address) get transparent access, and off-campus users must authenticate with their username/password.

This is a pretty basic and common situation, which I have dealt with many times using Apache.  But I cannot see how to do it with IIS.  I see how to set it for IP-controlled access, OR for username/password authentication, but not one conditionally dependant on the status of the other.

Worst case, we just require authentication for that directory for everyone, on or off campus.  But I feel sure that this must be possible with IIS, and that the problem lies with my lack of comprehension or experience.

Suggestions / web pointers / etc.?  Thanks!

-Edward Spodick, Hong Kong
 lbspodic at ust.hk