How to know what host is being called

I have a web app in C# and I want to redirect the app depending on the host being called on the site... for example if I call www.domain.com I want the app to be redirected to a folder called Site1 if I call www2.domain.com I want the app to be redirected to Site2. these are example names.. it's the same site different operations. All the DNS stuff is being taken care of, so no worries. I must say that www and ww2 are pointing to the same server IP address.

Thanks a lot guys..

(Sorry if the title is not too clear, I didn't knew how to ask the question properly)