Moderate nat problem?!

I have just recently started playing Halo Reach with my new 360 slim and though I can get on matchmaking I keep getting pop-ups saying that I may have problems with my connection and every time my friend tries to invite me to play I get a pop-up saying that I am unable to play do to having a moderate NAT. It says to go to to fix this but I have found nothing. I tried going through my router gateway and changing the setting but nothing. Can anyone help please?

This explains it pretty clearly as long as you are familiar with computers.

This also should help.

But I know for me, I used to have moderate NAT but never had any issues connecting to people, and my Network status was always Good, but after I opened my NAT my network status has been at Warning, fwiw.