How do you create a private area

Dear All,

I wish to separate my web site in to an area for all to see and three other areas only accessible to certain types of user.

I think that I have managed to confuse myself and would therefore welcome some input to set my thinking straight again.

1. Domain Mapping is as far as can tell not appropriate this is used for redirecting not controlling access.

2. I can see how it is possible to set up sub-domains and control access to those sub-domains by manipulating apache’s .htaccess file for the domain folder.

3. I can see how I could install wordpress on each sub-domain but this appears to me to be a duplication.

4. I want one installation of wordpress to work across the main domain and sub-domains and for access to the sub-domains to be controlled.

5. Therefore how do I tell wordpress that I want a particular page(s) to reside in the sub domain not in the main domain.

Any thoughts very welcome.