Hi,
For example when you have one public address outside servers sees you as one device but it don't have to be like that because you use NAT and private IP addressing.
When you sending packet from your pc lets say 192.168.0.10 port 6001 to server 155.123.0.1 port 80. Now your NAT changing you source address and source port from 192.168.0.10 port 6001 to your public IP lets say 80.100.100.1 and random port for example 36901. [here ends description how source nat working]
Now when server respond to you he uses your public ip and port that nat generated for you so destination address in packet from server will be 80.100.100.1 and port 36901. Now when your nat enabled device will receive that packet he will translate this destination address 80.100.100.1 and port 36901 to your private address 192.168.0.10 port 6001 and send this packet to you and this is example of destination NAT operation.
If you want perform only destination NAT because lets say you have a server with private IP then you need create static mapping on your NAT device so if you have web server then you need create mapping if there is incoming traffic for port 80 send this traffic to 192.168.0.1.
