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I just want to say thank you, this was a very concise explanation of a very complex concept.

I've been working with NATs for years and your comment helped me "click" and understand them at a different level.

Similarly, I found that OP's article provided an excellent primer on many concepts -- it certainly clarified the relationship between NAT and firewalls: that is, the latter being somewhat of an unintended consequence of the former.

Stumbling upon a great blog post that makes something click is always a pleasant experience.

Thanks! I guess I should keep blogging then :)

Me too. I’ve always wondered how a NAT knows where to route traffic. I figured it would use a lookup table, but I never know what the “keys” were. For some reason, using different ports for each device behind the NAT never crossed my mind! I knew it couldn’t be done by adding routing data to the packets (which is what IPv6 ended up doing) because that isn’t sustainable over multiple NATs. A port based routing with a table makes so much sense! It also explains why idle sessions are dropped.

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