If everything is inside your firewall (untangle is a good choice), you don't need all those static IPs. You can use one, and that would be very handy -- saves you from having to do DDNS (which isn't terribly hard, but it's just another thing to keep track of).
You assign one static IP from comcast to the "RED" NIC of the firewall. This is the only one you need/use (you might have other stuff going on to use the others, but not in this case for sure).
First thing to do: You should setup your subsonic PC/Server to have a relatively static address. If you're using DHCP, set the dhcp-server to "reserve" the IP that the subsonic PC has now, so it'll always give it out to only that PC...it's kind of like making the subsonic box have a static address, but you don't have to go through the motions on the side of the PC. I don't know what you're using for dhcp on your LAN...if it's in untangle, chances are it's dnsmasq, which is pretty easy to configure to do that, and untangle might have a page to just do it for you. Then you can rest assured that everytime you reboot your subsonic box, it'll receive the same IP Address from the dhcp server.
In the untangle box, you need to configure "Port Forwarding". I'll keep it simple and assume you're using port 4443 as your HTTPS port for subsonic (you can use virtually any, but that's referenced in the FAQ, so just following along with that). Forward (TCP) port 4443 from the "RED" interface to port 4443, IP Address of the Subsonic box (choose NAT if it asks).
Now, from the internet, just use the comcast, static address (which is really the untangle firewall) as if it were your subsonic host. Any traffic sent to that address, port 4443 will be forwarded to your subsonic host, where it can respond back. (Choose
https://<comcastip>:4443 if your subsonic client is asking for the server address.)
That's a lot to think about. Ping back if you get stuck. This should work and keep your subsonic box safe. AND, if I'm right, you can probably return some of those extra static IPs to comcast for a reduction in your monthly bill.
