The loopback securty check is a Windows feature that prevents access to a web application using a fully qualified domain name (FQDN) if an attempt to access it takes place from a machine that hosts that application. The end result is a 401.1 Access Denied from the web server and a logon failure in the event log. This security feature works well to prevent a server attack via reflection. However, this feature also interferes with the alternate accessing mappling feature of SharePoint Server [MOSS].