You should know what you actually need, maybe we can provied some help then.
Just because it can be done doesn't mean it has to be done!
Why would you want to separate your DC from the LAN clients? You would have to create firewall rules to allow traffic back and forth anyway. If not, then it's not your DC any more and if you do, the separation doesn't make any sense. At least with the information aboout your network we got so far (1 DC and 20 DHCP clients) this is not desirable to do.
KISS is your friend! (http://en.wikipedia.org/wiki/KISS_Principle)