I've noticed alot of people are having problems with there nat types, so i've found some useful information.

This explains why and how nat types are used for those who are interested.

http://en.wikipedia.org/wiki/Network_address_translation

This link shows how to achieve an open nat type with your ps3 by using a dmz.

http://community.eu.playstation.com/t5/PlayStation-Network/How-to-put-your-PS3-into-a-DMZ/td-p/10655314

For those who need to use port forwarding the required ports are

tcp 80,443
udp 3658

this information can be found on page 15 of your crysis 2 instruction manual