Hi,

I want to configure DNS server in 172.16.81.6. After the complete installation and configuration of DNS server, I want all my users to browse the internet by giving DNS IP 172.16.81.6 in their systems. Without giving the DNS IP 172.16.81.6 n their system, the users should not be able to browse the internet. For this, should I need to map 172.16.81.6 with the DNS IP given by ISP or just configuring DNS server in 172.16.81.6 will work?? Please help.

Not alot to help WITH. Use your internal DHCP server to set the DNS configuration on your clients...that's one thing DHCP does by itself.

However, you do realize that if you don't do anything else but that, your users can still access the internet, right?? All they have to do is use Google's DNS servers (entering them manually), or just put IP addresses of sites in their browsers, and off they go. Aside from modifying your DHCP server to point to your own internal DNS server, put a proxy server in place to prevent folks from getting straight out to the internet.

Hi,

As per your advice, if I use my internal DHCP server to set the DNS configuration on the clients, then the client system will take my internal DNS IP 172.16.81.6 automatically from DHCP and finally they will be able to access the internet only through my internal DNS IP right???????

and the second option, as you said, I can use the proxy server also.

No, that's NOT what I said. I said that you can use your DHCP server to SET your clients DNS server, that's all. Again, all that's going to do is make name resolution not work, but they can STILL manually use Google's DNS servers, or put in IP addresses instead of names, and get right out.
It's not an option. If you want to prevent users from getting to the Internet, filter websites, etc., you NEED a proxy of some sort. Just using internal DNS to restrict web traffic is very futile, and is just doing work for no real result.