If we are going to use any remote extensions or ITSPs, sooner or later we will need to deal with firewall issues. Unfortunately, the SIP protocol is not very friendly with firewalls and Network Address Translation (NAT).

SIP as a protocol design makes sense, if every computer on the Internet has a unique IP address. However, in many cases, the computers sit behind a firewall that performs NAT of some kind and is the only device with a publicly accessible IP address. This makes it quite difficult to manage sessions between machines as it requires a number of ports to be open to the machines involved in the connection. What this means is that we often have calling problems with NAT on our network, usually with a lack of audio or an inability to initiate calls.

To get around this, we will need to open some ports in our firewall to the TrixBox system. The following table lists the ports required for Asterisk to function efficiently: