iptabels -P FORWARD ACCEPT
iptables -A FORWARD --source NETZB --destination DSLROUTER -j ACCEPT
iptables -A FORWARD --source DSLROUTER --destination NETZB -j ACCEPT
iptables -A FORWARD --soure NETZB --destination NETZA -j DROP
iptables -A FORWARD --source NETZA --destination NETZB -j DROP