I don't know if I'm saying this right but, I have two computers in my LAN hooked up by a Ethernet cables to my modem/router zyxel pk5000z we were using theses computers to play together using Game Ranger. We were having some problems connecting to game rooms with a symmetric NAT. When the port forwarding did work it would only let me open ports for one ip adress for 1 computer it seemed. How do you open ports for all computers on your LAN?
If your online game requires specific ports to be opened, then you can ONLY forward those ports to ONE computer. That's just a limitation of using a NAT router.
The only way around it is if a) the game allows you to assign different ports for each user(so you can then forward those to another machine), or b) you have a second public IP. Of course, a second public IP is only usuable if you have a second modem, and perhaps another router if you want that connection firewall’d as well.