How IIS distinguishes between Asp.net and Asp.net MVC req.?

Question

While installing .net framework on server .aspx, ascx.... etc. extensions are mapped for asp_isapi.dll. So, when request arrives request for .aspx is given to ISAPI.dll (here we treat request as ASP.NET request.)

                                           But in case of asp.net mvc request how and where it is getting detected as the MVC request ?.