Re: DMZ the server...with the firewall off. Uhm...."not good!" Even with the firewall turned on, a Windows computer hanging out on the internet, outside of NAT...I'd want to format that if it's been sticking outside the firewall for more than a few minutes. You should at least check its logs, look for odd accounts, change Admin password again, run a full scan on it.
Here's an example of a setup.
Server IP 192.168.10.11
255.255.255.0
gateway 192.168.10.1
DNS 192.168.10.11
Now setup the DNS forwarders...start...run...DNSMGMT.MSC this brings up DNS management.
Right click server, properties, forwarders tab, this is where you set the public DNS servers that the server turns to for requests for zones outside of its own internal AD. Typically you enter the ISPs DNS servers here, or...I actually prefer to use OpenDNS's servers for increased security..208.67.222.222 and 208.67.220.220. Some people put Google DNS..8.8.8.8 and 8.8.4.4
Now for DHCP for the LAN, create your scope, 192.168.10.0
I typically exclude 192.168.10.1 to 192.168.10.99 so the dynamic handout range starts at .100 on up for workstations
I set servers in the teens, printers in the 20's and 30's, managed switches 'n NAS's in the single digits, etc.
DHCP hands out the address, subnet, gateway, and the DNS it will hand out is the servers IP, 192.168.10.11.
This is an OLD article I did back quite a few years ago on my Small Biz server at home, while it's outdated, the principles are still true
http://www.speedguide.net/articles/server-based-network-guide-1660