How to use subdomains as filters? MU


I am developing a site using subdomains MU, and works perfectly.

This site will be a kind of directory, what I need and do not know if possible, is that i can use some subdomains in order to make such a filter. ej take the subdomain and display only the sites in this area in the main blog (i already have the filter and so), is this possible?

Thanks in advance