This should help: NAT Configuration
This common design is required when the site has a single public IP addressfor a number of services that are provided by different systems internally. This is also referred to as port forwarding or Virtual IP’s (VIP’s). Note that in this scenario the internal web server is running on port 8080 rather then on the traditional port 80.
So in the original packet tab, you define the service as the destination port the external users will be connecting to. In the translated packet tab, you check the destination address translation box, and define the destination IP and port. So the original packet is 1.1.1.1:1234 and then you translate it to 172.16.1.1:80.
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