Firstly, I wasnt sure if this was the appropriate forum section to post this, or in off-topic. But seen as its about Xbox 360 specifically, I posted it here (apologies if it should be in off-topic) and there tend to be many knowledgable people here, so hoping to find some help :)

Ok, so me and my brother both recently have a 360 - both running two individual Xbox LIVE accounts. They are both connected to our router. But we have been having alot of problems when both are running at the same time, and connected to the internet.

If one of us uses the Xbox and connects, there is no problem. But when the second one does the same, that system recieves messages about the NAT being closed and it may cause connection problems. This only happens when we try to use two at the same time, and the one affected by the problem is always the second one to connect.

I havent had problems running two different gaming machines with this setup before, its only with the two Xbox's at the same time. It causes many problems, such as when one of us disconnects, so does the other. Random DC'd, more lag.

Can anyone help? is there something I need to setup so we can run both?

The router is a Linksys BEFW11S4 (its a bit old now, but always worked fine until now). I know some of the basics about the settings etc, but by no means am I an expert.

Any help appreciated :)

They should autoconfigure with upnp. The first system online will redirect the default port 3074 UDP, subsequent systems will redirect on a random port. This can be confirmed on the router's UPNP mapping page if it has one, and also on the 360's information page in settings.

Even though the random port reports as not fully open, other 360s can connect to it without problems. So don't worry about it.

Well the problem is with your router. Just google how to open NAT and it should show you how to fix the problem.

If your NAT is closed in general you will get a worst connection and experience playing games as people will have troubles connecting to you as you will have troubles connecting to them.

The weird thing is that is sounds like your NAT only closes when you have the second 360 on which shouldn't be the case, if the NAT it is closed it is always closed and vice versa.

My friends have two 360's connected to their router and play online together all the time so I know for a fact it can work without a hitch. 

I have a Linksys wrt610n and this is what I had to do.

I had to go to the the cisco network magic map. Click on the router, then click set up or manage device. Put in the username and password, which both default settings are admin.

Go to application and gaming.  Since you are trying to make two Xbox consoles use those ports and you are trying to connect to a server, what you need to do is to use Port Range Triggering.The ports are 53, 80, 88, and 3074 then enable. You can't use Port Range Forwarding since it will only set those ports into listening mode to the IP address you set it. So if you use Port Range Forwarding it will only be open/available to one console(the one using the IP address).

You need to use PORT RANGE TRIGGERING. Disable Port Range Forwarding (if you have enabled it for the one xbox) also disable DMZ. You need to enable UPnP as well if your Linksys router has this option (other model doesn't have this option but it is said to be enabled in default settings according to their tech support). You can find this on this router in storage and then click on media server.

To solve the lag problem set your MTU size to 1364. This settings will work even if you have one or multiple consoles running behind the router.

There's nothing you can do about it really. You can only forward port 3074 to 1 device at a time, so whoever connects first will get their NAT status as open and the other will be strict.

hard wire a PC to your router and open up any web browser, the address to your router is probably http://192.168.1.1

go to the setting that says NAT from there look for the setting to enable UPnP (Universal Plug in Play) Dont forget to save settings. This should take care of your problems.

I tried the enable UPnP option, but when that was turned on my interenet kept dying completely.

It was quite strange, as nothing would work. So I turned the power off on everything, then re-booted and my connection restored. But it only lasted about 20 minutes and again everything died. No web pages would load, I couldnt even connect to the router through the browser when it happened. Even more strangely, I tried restarting my PC, but it wouldnt boot correctly. It booted up to the desktop, but all the desktop icons were gone, and all I had was my mouse pointer on the screen on my wallpaper unable to do anything. I tried restarting the PC a few times, but each time it booted up it did this. In the end I had to again turn all the power off, then reboot and it worked and I had to turn off the UPnP option and since then everything works fine again (well, excpet the 2 Xbox's thing)

Try forwarding the xbox live port to one xbox. When you have both xboxes online the one you have forwarded its live port will be able to become host and have its NAT open but the second one won't.

If you play the same game and you become host the second xbox in the house will have very low ping since it will be connected to your xbox so both of you will have very low ping.

Now if you are both connected, play different games and want both to be able becoming hosts it cannot be done because all xboxes communicate on the same port and only one xbox can have that port open.

Here are a couple of links with varying solutions

http://www.gamespot.com/pages/forums/show_msgs.php?topic_id=25996060

http://www.pistonheads.com/gassing/topic.asp?h=0&t=485537

one of the things they mentioned was the DHCP setting in the first link.

wow, if you have to manually turn on upnp and it's not enabled by default, it sounds like an old router. That setting shouldn't change anything with your internet connection, so it sounds like there's something glitchy with your upnp. Sounds like you need to upgrade.