This is an ongoing problem, and one that I have had for years. Even though I have my router fine tuned to every xbox spec I've come across, and from XBL help over the phone, and even though my friends also have the same set up, we can't connect to each other. I have had this issue for years, and I thought my troubles were over a few months back when everything started to work just fine, albeit some friends were still unable to join my parties. Ever since the most recent update, nothing. Everything has gone downhill, and nothing works. I'm lucky I can still send my friends messages without issues, and they are only ever sent to inform them that yet again, I can not connect to their party. I have seen countless people stating the same recently, and they are all suffering as a result of the new update. This is yet another problem with the service that i now have to deal with. There is no point calling for help, as it is a continuous circle of blame passing. It's my ISP, it's NAT, ports, DMZ, etc. etc. etc. Well now it is not. If I have to look at adverts on this paid service, I would at least like to be able to use it with ease. This has never been the case. Please, respond with something other than 'Your router isn't set up properly'.
Please read:- forums.xbox.com/.../811997.aspx
And post back your details in full (the bits that are in bold on that post) so that people may help
If you are wireless can you also state whether you are using the white or black external adapter or the Xbox 360S Internal adapter
XBOX LIVE Connection Advice NAT/IP/DNS/MTU/ICS/Bridging Click 4 Help