Windows server 2008 is not connected with the internet

Hi I have a windows server 2008 which is acting as the only DNS and DC in my network. But the server is not connected to the internet. So that im using the public DNS servers in all the client machines.

Servers TCP/IP:
IP:192.168.x.200
subnet: 255.255.255.0
Gateway: 192.168.x. 400(The IP address of the modem provided by ISP)
Primary DNS:192.168.x.200(I just pointed to itself)

Client Machines Tcp/IP:

IP:192.168.x.x
subnet: 255.255.255.0
Gateway: 192.168.x. 400(The IP address of the modem provided by ISP)
Primary DNS: Public DNS Provided by ISP

If i point the client machines to my local DNS the client machines are not connected with the internet.
Im just a beginner. Can any one help me out of this.

Regards,
Prabu.