IIS is a dedicate webserver taking care of all the application inside it. it maintains all the applications inside the dedicated application pools and these application pools will be responsible for routing the request of client. when client request a url from browser ,the request comes to IIS which work on these requests on two levels .. kernel level and user level. kernel level has a http.sys file which is respnsible to take the request forward to application pool with the helper present at user level i.e .web activation server . WAS look on the request and pass the request to a dedicated application pool ,the pool will further forward this request to perticular application's worker process.this worker process will look the extension of the request and base on the request ,routes the request to httpmodule/set of http module(http pipeline)these modules will do their working like authentication/inserting any preprocessing logic and then send the request to appropriate httphandler which in turn after authentication will design the respnse and sent to client via the same root.