Help needed for Windows Authentication

Hi every one,
I set IIS to use Windows authentication.

when I use URL like this

http://servername/sitename

then it works (I wont get any logon name). I can reffer
logon Name using servervariables("AUTH_USER").

But the problem is,
when I use
http://servername.domainname/sitename

then it displays windows default logon dialog box.
I have to enter my username and password.

my question is
why I am getting logon screen when I use fully qualified
servername (servername.domainname)

Is there any way that I can configure IIS not to display
logon screen when I use
servername.domainName


Expecting ur Cute Replies ...