How to redirect blog.mydomain.com to mydomain.com/blog?

I use the following code for redirection

Code:

        
string strServerName = (Request.ServerVariables["SERVER_NAME"]);

        if (strServerName.Trim().ToLower() == "testapp.mydomain.com")
            Response.Redirect(@"/testapp"); 

Unfortunately this code redirects to url testapp.mydomain.com/testapp
which screws up my MVC application. How should I change redirection, so that url will be testapp.mydomain.com?