Comment by digitalsushi

There are like 4 different NAT port translation models. In some of the models, the NAT algorithm will actually preserve the ephemeral port number in the NAT mapping. This falls apart quickly when you have a few NAT clients. But it can make legacy software work a lot better. We have weeded most of that software out at this point, or at least provided application gateways that inspect the content of the packet and tweak little things - consider how you need an extra kernel module for FTP and SIP and stuff.

The models are detailed nicely in this diagram. https://en.wikipedia.org/wiki/Network_address_translation#Me...