How to configure NAT to act as port restricted?

I am working on a project which require me to test with different type of NAT. I am using a laptop which is acting as Linux router. Due to some reason it is not able to correctly identify NAT. Here is section of code that I am using
iptables -t nat -A POSTROUTING -o $WAN -j SNAT --to-source 192.168.10.203 iptables -t nat -A PREROUTING -i $WAN -j DNAT --to-destination 192.168.25.10 iptables -A INPUT -i $WAN -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i $WAN -m state --state NEW -j DROP
Is there a better way?

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