3 lines for linux nat

iptables

-A PREROUTING -d 172.17.72.6 -j DNAT --to-destination 10.21.1.6
-A POSTROUTING -s 10.21.1.6 -j SNAT --to-source 172.17.72.6
-A FORWARD -s 172.17.72.2 -j ACCEPT