After years of frustration, I finally figured out how to get the best performance for multiple Xbox 360s on one LAN. I have tried several routers from different manufacturers; I finally found the DLink DGL-4500 Gaming Router.
Here is what I did to set it up:
1: Setup DHCP for your LAN and set aside static IP addresses outside the DHCP range for each XBox 360.
2: Goto Advanced, Special Applications
-- I added 4 rules:
Xbox 1 - Trigger Port (Both) 53
Firewall Port (Both) 53
Schedule (Always)
Xbox 2 - Trigger Port (Both) 3074
Firewall Port (Both) 3074
Schedule (Always)
Xbox 3 - Trigger Port (TCP) 80
Firewall Port (TCP) 80
Schedule (Always)
Xbox 4 - Trigger Port (UDP) 88
Firewall Port (UDP) 88
Schedule (Always)
3: Goto Status, WAN
-- Write down the IP Address you are getting from your ISP
4: Goto Advanced, Gamefuel
-- Add Rule for each Xbox
Name: Xbox 1
Priority: 1
Protocol: Both
Local IP Range: 192.168.1.100 TO 192.168.1.100 (This is the IP of the 1st Xbox you assigned)
Local Port Range: 1 TO 65535
Remote IP Range: 172.175.16.233 TO 172.175.16.233 (This is the IP of your Router's WAN IP)
Remote Port Range: 1 TO 65535
Name: Xbox 2
Priority: 2
Protocol: Both
Local IP Range: 192.168.1.101 TO 192.168.1.101 (This is the IP of the 2nd Xbox you assigned)
Local Port Range: 1 TO 65535
Remote IP Range: 172.175.16.233 TO 172.175.16.233 (This is the IP of your Router's WAN IP)
Remote Port Range: 1 TO 65535
Name: Xbox 3
Priority: 3
Protocol: Both
Local IP Range: 192.168.1.102 TO 192.168.1.102 (This is the IP of the 3rd Xbox you assigned)
Local Port Range: 1 TO 65535
Remote IP Range: 172.175.16.233 TO 172.175.16.233 (This is the IP of your Router's WAN IP)
Remote Port Range: 1 TO 65535
4: Reboot
5: Goto each Xbox 360 and assign a static IP address.
Now the lag is virtually gone and the online gaming experience is so much better.