restrict access to application in web.config

Just wondering if there was anyway you could restrict access to an application (folder made into an application in control panel) using something like a web.config file that looks at the servervariables("SERVER_NAME") for a match to a string, if it matches, then access i given and u can browse the pages. otheriwse, it just redirects user to an error page?

or a global.asax file at the root?

cheers