.NET Magazine - Manage Web Sites With ASP.NET: "Versioning ASP.NET Applications
The .NET Framework and ASP.NET enable you to run multiple versions of ASP.NET on the same server to preserve compatibility with applications built against a previous version of the .NET Framework SDK. When the .NET Framework is installed on a server with an existing version installed already, the ASP.NET applications on the machine are updated automatically, by default, to use the .NET Framework's newest version. There are exceptions for applications that are bound to a later version of the runtime or an incompatible version. Although the .NET Framework is designed with backward compatibility as a goal, you might need to configure an ASP.NET application to use an earlier version in some cases. Or, you might need to run two applications on the same server targeted to different versions of the framework. Note that when you run IIS 6.0 and Windows Server 2003, you can enable ASP.NET in the IIS management console after installing ASP.NET. IIS 6.0 provides a new Web Service Extensions folder, enabling you to disable and enable IIS functionality selectively.
The ASP.NET ISAPI version associated with an application determines the runtime version the application uses. If you configure the application to use the earlier runtime version, the component is redirected automatically at run time to use this version. If the runtime version used to build the component is installed on the server, you can reconfigure the application to use the later runtime version. Microsoft designed ASP.NET to support different runtime versions, so when an application uses assemblies built against different versions of the .NET Framework, the runtime version associated with the application determines which version of the .NET Framework assemblies the application and all its components use.
For example, if"