The ServerName directive sets the hostname of the server; this is only used when creating redirection URLs. If it is not specified, then the server attempts to deduce it from its own IP address; however this may not work reliably, or may not return the preferred hostname. For example:
ServerName www.wibble.com
would be used if the canonical (main) name of the actual machine
were monster.wibble.com
.
See Also:
DNS Issues
UseCanonicalName