There is nothing in the specs that I have seen that requires you to have it right up to the internet to work, so you can put a firewall, a router between it and the internet if you feel the need to.
Actually, if your going to use the asterisk VOIP part for making and receiving phone calls, you'll need to configure your router/firewall (if you use one), and parts of your asterisk configurations. You may have to get sipx ser or another proxy setup.
And it can be a real headache if your multiple nats.
I maybe able to help if you have any problems getting asterisk working right, and modifying it to your wishes.. Let me know.