NAT type is Moderate, how to change it to Open?

My 360 is saying that my router's NAT type is Moderate, and I've been trying the canned solutions (making sure UPnP is on, etc.), plus updating my router's firmware, with no luck. From what I found from googling, I think I might have to change my port forwarding settings, but I can't forward to my 360's IP address, because the one the 360 automatically obtains isn't compatible with the router settings. ie, it should be aaa.aaa.a.a, but instead it's aaa.aaa.b.a, and I can't change the errant digit in my router's config page (it presumes that the right digit should be in that place, and only lets me enter the last digits). I would manually set the 360's IP, but I can't connect at all that way.

Any help?