ASP.NET Page Execution
In the “classic” ASP model, the ASP processor was implemented as an ISAPI extension (asp.dll). ISAPI is a programming interface that developers can use to extend the functionality of Internet Information Server (IIS). The asp.dll simply supports specific interfaces and types to be an Internet Server API (ISAPI) extension. ISAPI extensions are associated in IIS with certain file types. For ASP, the .asp file type is associated with the asp.dll.
When an HTTP Request is received by IIS, it looks in its Metabase to determine which ISAPI extension is registered to handle this type of file. IIS then routes the HTTP Request to the appropriate ISAPI extension (that is, asp.dll).
The asp.dll is now responsible for handling the request. ...