Help with networking thru macbook

Hi everyone! I need a bit of help with my network. I recently managed to connect up my xbox 360 to the internet by using the internet sharing feature on my macbook. However, xbox live states that my "NAT" is strict, and that's a bad thing. Does anyone know how I can change my "NAT" without changing my existing set-up? My network is a wireless router connected to my dsl modem, and my macbook is the only mac on the network; there are two PCs. Can anyone help?

 

Its a lower-end dlink model. I think it's the WBR-1310. I'm on AT&T DSL, if that helps.