There are a set of parameters in the SIP profile called ext-sip-ip and ext-rtp-ip. These parameters are used to supply information about how to behave in regards to external IP addresses. The default configuration defines both of these parameters with auto-nat. This is used together with the NAT router control feature we were just talking about to just do the right thing. Some of us will not have a router that supports NAT-PMP or UPnP, or worse, it will claim to support one or the other and then not work at all because either it's inherently broken or due to some other deadly combination of the pitfalls.

We can disable this functionality when we start FreeSWITCH by supplying the -nonat command-line option. With this option disabled, we still have some tricks up our sleeve. You can put an IP address in either field when you already know what it is, for example when you have a static external IP address. Better yet, you can set it to autonat:x.x.x.x (where x.x.x.x should be replaced with your public IP) so it uses your known external IP and still does a bit of dynamic magic.

ext-sip-ip="auto-nat:194.20.24.1" 
ext-rtp-ip="auto-nat:194.20.24.1"