When the web server receives the request for the file with the extension 

.aspx, it forwards it to_____________

Requests for ASP.NET resources (for example, *.aspx, *.asmx, *.ashx) are passed on to the ASP.NET ISAPI extension. This system component is a Win32 DLL named aspnet_isapi.dll. The ASP.NET extension handles various resource types, including Web services and HTTP handler calls.