Windows authentication and no-ip

Hi,

I'm using Windows XP Professional and have a web site for which I want authentication used. Using Basic Authentication always works (but I don't want to use this), using Windows Authentication does not work when browsing via a dynamic DNS (no-ip.com).

Basic authentication
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http://machinename/test.htm - prompted for username/password, works fine
http://machinename.no-ip.info/test.htm - prompted for username/password, works fine

Windows authentication
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http://machinename/test.htm - automatically logged on, works fine
http://machinename.no-ip.info/test.htm - prompted for username/password, CANNOT logon and after three attempts get the 401.3 error

I've searched the web for related stuff and found nothing that helps. (SetSPN was promising since the symptoms seem the same but I'm not in a domain).

What am I overlooking????

A quick further check removed the no-ip redirect from the equation, simply entering:

http://001.002.003.004/test.htm - prompted for username/password, CANNOT logon and after three attempts get the 401.3 error