A little bit of background first. I've been a subscriber to XBox Live! since its launch in November of 2002. For a while, I was on DSL (at my old apartment), and had a nice static IP dedicated solely to the XBox. Later, I bought a house and could no longer get DSL, so I switched to cable. No more nice static IPs, but I paid for a second dynamic IP and strung a looooong ethernet cable across my entire house and hooked up the XBox that way. In both of these cases, life was good. Of course, I couldn't just leave well enough alone. I was annoyed by the long ethernet cable, and Microsoft had just shipped the XBox Wireless Adapter, so I bought it. Now, a bit of background on my wireless setup: I use a Linux machine to do Network Address Translation (NAT) for my internal network (a wired desktop, a wireless laptop, a wireless TiVO, and now a wireless XBox). I've used a similar setup since college, oh so many years ago (I believe the first time I set this up was 1998). Anyway, back to the story. I plugged in the XBox wireless adapter, set it up, and things worked great. The XBox could connect to Live!, and I could play games. Then I noticed that there were some games I couldn't connect to. I lived with it, because there was usually an abundance of games to choose from (btw, my poison of choice is Project Gotham Racing 2 (PGR2)). Now that the game has been out for more than a year, the number of available online games or "rooms" has dwindled, and the "This session is no longer available" message is much more annoying. When there are only four or five available games in the list, and I can only connect to one of those if I'm lucky, my gaming time is spent more on searching for games than actually playing.
Well, I finally decided to do something about it. I was told to check out this page to help diagnose my problem. The key metric here was the "NT" value, or NAT Type. My XBox reported that value as 1, or "open", which means I shouldn't have any problems connecting to online games. And yet, I was. The description of NAT Type 1 is as follows, and gave me my first clue:
Open NAT means that either the port assignment policy is minimal or the router has a fully compliant version of UPnP (Universal Plug and Play) enabled by default.