When you use the fully qualified domain name (FQDN) to browse a local Web site that is hosted on a computer that is running Microsoft Internet Information Services (IIS) 5.1 or IIS 6, you may receive an error message that is similar to the following:

For example: logging to http://server/panorama/welcome.htm - OK

the adress http://server.domain.com/panorama/welcome.htm - 401 error

HTTP 401.1 - Unauthorized: Logon Failed

This issue occurs due a loopback check security feature that is designed to help prevent reflection attacks on your computer.

Therefore, authentication fails if the FQDN that you use does not match the local computer name.

Please refer to the Microsoft KB article for details:

You receive error 401.1 when you browse a Web site that uses Integrated Authentication and is hosted on IIS 5.1 or IIS 6
http://support.microsoft.com/default.aspx?scid=kb;en-us;896861

• Not Applicable (This is an IIS issue)