MW3 NAT Type strict and what i done to fix this was really easy
In your ps3 network setting perform internet test and check below
NAT Type 3 = Strict
NAT Type 2 = Open
If you have type 3 then what i did was input a manual ip address in the ps3 settings if this don't work for you then connect to your router and turn on Universal Plug and Play
Universal Plug and Play (UPnP) is a technology that enables seamless operation of a wide range of games and messaging applications.
Reboot your router after. Then try MW3 game again this time your NAT should say open.
Once its open you can host games and people can join your MW3 party.
If this still does not work for you then post up your problems with NAT Type
More official help can be found here http://www.infinityward.com/nat.php
Log into your router and turn on UPnP (Universal Plug and Play), most routers have this option in the Administration tab.
The goal is to have an OPEN NAT in order to host Spec Ops games and get a wider selection of MP matches to join call of duty modern warfare.
Also ensure that under your security options that Anonymous NAT Redirection is allowed and is not filtered. This may also be referred to as 'Filter InternetNAT Redirection' - in which case you want this disabled.
For more information on enabling UPnP on your router, visit your router manufacturer's website. A few of the most popular router web sites are listed below:
•Linksys: Support and Services
•Netgear: Support Home Page
•D-Link: D-Link - TechSupport
•Belkin: Belkin : Support
MW3 NAT Type Strict *Fixed* I pre-ordered this game back in June and it arrived a day early so happy days and my nat is now fixed!