Windows 2008 DNS NS ip changed by itself

Hello,

I used VirtualBox to set up a Windows 2008 DNS server. I have one public IP and I am behind a router. The 2008 server is assigned with the internal IP 192.168.1.100 and the router is configured to forward any HTTP request to this IP.

The problem now is that I have set the IP for NS record in DNS to my public IP, it stays that one for a couple minutes then is changed back to 192.168.1.100 by itself. If I restarts the DNS service, then the IP is changed immediately. The IP for www record is fine, so accessing the website with www before my domain works.

I have set up another DNS as the secondary dns server and named is ns2.mydomain.com. I assigned the same public IP to it stays.

Does anyone know why the IP for my first dns server changed back to the internal IP automatically after the dns service is restarted?

Thanks a lot.