I think the suggestion to try it at someone elses house is valid. Unless you have another router you can try. I guess it's your parents' router? I guessing the router does have NAT but for some reason the PS3 cannot tell what type it is.
Google your router model. Is your router C2 or C3 hardware version (it says in the top right corner of the web configuration page)? These two have different manuals. C3 has an option to enable/disable NAT but makes no mention of uPnP. C2 does not have an option to isable NAT (and I'm not sure why you'd want to) but it does mention uPNP.
Sorry I couldn't be more help.
i dont know why u have to open nat type. shud be as easy as that bt i find most new zealanders are gna giv up on ps3 cos of this problem with mw3 on ps3. why has activision made it so hard. i dont blame people for leaving ps3.
from Ontario you might get more help in your own timezone on the US forum http://community.us.playstation.com/community/supp
Also, have you tried resetting your modem and router?
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