How do you open your NAT?

I play Modern Warfare 2 on the PS3 and can join with alll of my friends except my best friend, it can get annoying where we want to play but can't unless someone else is hosting, my question, how do you open your NAT so we can play, both of ours are Moderate but we still can't connect I have a belkin wireless and he has quest, is that the issue? I have already tried enabling the UPnP and even entered the PS3's address into the DMZ and still nothing CAN SOMEONE HELP? Btw- when i did these two things and checked my PS3 it was still Moderate not OPEN.