ASP.NET was originally designed as an open framework. That is, many of the modules and components that make up ASP.NET can be extended, modified or replaced to suit your particular needs. In ASP.NET 2.0, the extensible nature of the framework is clearly illustrated with the new HTTPHandlers and HTTPModules that are now a standard part of the framework.

The Request Pipeline:

In ASP.NET, requests are passed from the Web server through an Internet Server Application Programming Interface (ISAPI) filter and on to the actual ASP.NET runtime.

When IIS receives a request, the extension is mapped to an ISAPI filter according to the IIS settings. The extensions .aspx, .asmx, .asd, and others are mapped to the aspnet_isapi.dll, which is simply an ISAPI filter that launches the ASP.NET runtime. Once a request hits the ASP.NET runtime, it starts at the HTTPApplication object, which acts as the host for the ASP.NET Web application. The HTTPApplication object:

1. Reads the machine and application level configuration files.