I have a linksys router, & i have enabled UpnP & DMZ and opened the ports but its still not working.
What ISP and kind of Internet? DSL? Cable?
If it is DLS, do you have a modem/router combo from your ISP? If so, that may be the culprit. You may need to set that to bridge mode, and enter PPPoE information into your router.
Can you explain how to enter bridge mode please?
It's the same way you've been accessing your router, but instead, you access your modem. The modem should have an IP Address printed on it, and it may have a username/password also to get in. Or it may let you into the page, then when you try to configure it asks for a username and password. It varies by modem/ISP.
Once inside the access page, you'd want to look at settings that deal with WAN, or Connection. If you look at it, it may be setup for PPPoE (and have username/password info). You should be able to change the PPPoE to Bridged Mode.
Then, back in your router, you would want to set it's WAN/Connection to PPPoE mode, and you'd want to enter the username and password assigned by your ISP (you may have to call your ISP to get this info, or it may be your master account holder username/e-mail and password).
The exact settings/name varies, but you're looking for PPPoE and Bridged Mode on the WAN or Connection. You want to put the MODEM into Bridged mode, and put your ROUTER into PPPoE mode.
You don't need no special cable, you can be wireless. What you'll need is: your ps3, a computer, and have a router connected to your modem. Open your web browser on your computer and in the address bar enter your IP address " you can find this in you network settings on your ps3 or computer, once you enter it it will bring you to your router settings if it ask you for a user name and password enter: admin for the name, for the password enter: 0123, 1234, adm123, password, or you can look up the default passwords for your router on the web. once your in you' have to set the settings for the connection your ps3 is on, to make that easyer set your ps3's network manualy and change the last number of your ip address to 48 or any number you like this way it will make it easy for you to find the connection that belongs to you ps3, now for the settings they very from routers. but once you the idea you should be able to set your nat type to what ever you want it to be. After this is done restart your ps3 and run a test on your network once its done put in your game and enjoy.