IPTABLES chain need help for restriction

IPTABLES chain need help for restriction

I have two network cards
eth0:- 172.16.91.70 (External Zone)
eth1:- 172.16.85.70 (Internal Zone)
I have clients in my Internal Zone which can access the external network.
The ping goes fine and can access external net. Now I want to restrict the
access for the internal zone. The external zone connects to another
network that is 172.16.81.0 and that connects to 172.16.79.0
Now I want my internal zone be able to access complete 172.16.91.0
network, drop packets from 81.0 network and access 79.0 network. But I
also don't want it to access internet because the squid proxy server is in
81.0 network.