Demystifying NAT settings in FreeSWITCH

Now that we have reviewed the common pitfalls of NAT, we can go over the various types of NAT situations that you may encounter. There are several technical differences between the various implementations of NAT as well, but we won't focus on that because you'll probably fall asleep and miss the point of the chapter which is learning how to use FreeSWITCH in a NATed environment. Basically you will probably be in a situation where either your phone or PBX is behind NAT talking to a SIP endpoint that is not behind NAT (or vice versa). Even worse, you might end up in the dreaded double-NAT situation where both sides of a connection are independently behind their own individual NAT routers at the same time. A double-NAT scenario looks like the following diagram:

Let's start with a sane, yet challenging situation where you have a phone at your house that can't understand NAT and you want to register to your FreeSWITCH server that is on the public Internet...