IIS error: The page you are requesting cannot be served because of the extension configuration.

If you have installed ASP.NET 4.0 runtime on your system before installing IIS, you may run into this error while trying to run your ASP.NET web projects locally configured in IIS:

HTTP Error 404.3 - Not Found

The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.

To fix this error, you can simply register ASP.NET 4.0 runtime with IIS using these steps:

1. Open VS 2010/2008 (if you are using ASP.NET 3.5) command prompt.

2. Type in this command: aspnet_regiis.exe  -i

3. After the command has finished running, you will get a message saying: Finished installing ASP.NET (version).

Your site should work now.