You can easily setup a nat-ip-forwarding firewall for a router here, too.
You'll need to copy and paste the resulting page into a text editor and save it.
chmod u+x firewall.sh
I'm not sure exactly what you are trying to do. If you open a port through the firewall on your linux box, you'll be able to access it from the lan. If you want to access it from outside of your network, you need to open the port in your router settings and forward it to the ip of your server. In a stock linksys router, these settings are under "Gaming and Applications". The firewall on the linux box won't care if the connection comes from lan or wan. (Unless you configure it that way )
If you're going to run outside services, you might want to look into tcp-wrapper, too.
What did you say your external ip was again? LOL
At least go through this section