I cant change my nat type and am having problems playing games online with my playstaion 3

I take this is your type of network you have. NAT (Network Address Translation) only job is make all the IP's of you LAN to be able to use the one IP From the ISP. For example, you can have 3 Pc's connected to router (192.168.1.1 to 192.168.1.3) with only one connection to ISP, and address like 67.123.43.200 (Address change upon ISP location).
In order to the game communicate between PC ans Internet have to configure Ports to connect to. Thanks to UPNP (Universall Plug and Play) this configuration is made automatically between the PC and the Router and simplifies those configurations.

Things to check:
The router have UPNP and is on.
There's a Firewall interfering with the game.

You will need to change the firewall settings of your router. If your Internet service provider is DSL then make sure that modem is in bridge mode. How to change NAT type to open

Try doing a factory reset on the modem/router, also do a connection test on your PS3 and make sure that your NAT TYPE is 2 anything other than that will give you problems playing online. Let me know if you need help with getting your PS3 to a NAT TYPE 2.

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Nat 3 means that your PS3 is having problems accessing the internet connection. The best way to resolve this is to not play online using wifi just connect your PS3 directly into your router via an Ethernet cable.If this doesn't work call your internet provider and they will be more than glad to fix your problem.

Nat 3 is the result of being hooked into a router without being properly setup for data transfer. essentially it is restrictive.

There are 3 ways to change your Nat type to type 2 or 1. (Nat type 2 meaning that it is properly setup without restrictions behind a router, and Nat 1 being a direct connection to the Internet).

1.) Hook the PS3 directly into the modem will result in a Nat type 1, but of course that means disconnecting everything but the ps3 and that's an unnecessary hassle.

2.) Going into the router settings and Port Forwarding the PS3's necessary ports to the Ps3's ip address.

3.) Probably the simplest (on most routers) and most effective is to go into the router and set the PS3's ip address in the DMZ (DeMilitarized Zone), essentially the router doesnt have any say from what comes and goes to the Ps3, which is good so the connection isn't hampered.

just the ISP service local has the password for the modem ask they for change the settings for playing games online

what is your nat type which is located on the main menu of cod mw2 online?

your routers parental level is probably on 9 mate, you cant play any games like 15 0r 18 online, get your parent to change the parental level.

thanks for help , the error was from user that have viruse on his pc , thanks again

If the problem has something to do with NAT firewall in your router, then you may need to open certain application ports for your XBOX in the router. You may check this link from Microsoft http://support.microsoft.com/kb/908874 that talks about NAT firewall and XBOX live.

Hope this helps!