Please, correct me if what I say is wrong:
- the 172 network is you internal network;
- the 192 network is your access to the public internet;
- the 10 network is for any public service you want to offer.
If the above statements are right, you need a firewall with DMZ (Demilitarized zone).
You can find several samples, tutorials and scripts for this in the following link www.linuxguruz.com/iptables
Have a nice week end!