How to prevent outsiders from accessing ASP.Net intranet website?

[JB]

Hello Community

I have a main server at one location and I have 10 other remote locations
each containing one server. The main server contains the ASP.Net website
which is the intranet. Using IIS a user at any of the 10 remote locations
can access the ASP.Net website but how can I prevent anyone outside of the
10 other remote locations from other accessing the ASP.Net website at the
main location if they know the url to the ASP.Net website on main location of
the intranet?

Thanks
Jeff