Multiple versions of the .NET Framework can run on a single computer at the same time. This means that you are able to specify which version of the Common Runtime Language you’d like to use on an application level. The tool that helps you in this is called aspnet_regiis.exe.
When you install a more recent version of the .NET Framework on a computer, all of the ASP.NET applications installed will be configured to use the newer version, by default. You can then use the above mentioned aspnet_regiis.exe for changing this behavior.
When multiple versions are installed in your computer, each version has its own ASP.NET ISAPI extension (Aspnet_isapi.dll). With the aspnet_regiis.exe tool, you are able to define which version of the DLL you’d like to use on an application level, in IIS. The IIS Registration tool then sets your application’s script map to the appropriate DLL.
A script map is built from three elements: a file extension, an HTTP verb, and the version of the ASP.NET ISAPI extension, with which they’ll be associated. You can set the script mappings in an application (or application-pool) level basis, in IIS.