iv notcied whilst playing cod4 that i cant join my mates invites due to the fact i have nat 3, now i have tried multiple online self help step by step etc etc but none have worked.

I have a router and a modem. both dlink

Router- DI-624
Modem- DSL-302G

The ps3 is connected by ethernet to router, which is then connected by ethernet to modem

Im bascially network illiterate so you'll have to explain how to change my nat in laymen's terms.

cheers

jon