So your DSL modem is also a router and the PS3 is plugged directly into the modem? I don't know how much you know about networking, but the easiest way would be to simply put your PS3 in the DMZ. If you know how to access your router's settings that option is likely in there. Though that may also require you to manually set your PS3's IP address since that how many routers assign the DMZ. Your other option is to open up ports on your router.
Check out this link for more information:
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