That notice is sent to tell you that the server is automatically masking your hostname so that other IRC users on that network cannot see it. Rizon-B50A43.dsl.pltn13.pacbell.net is the masked hostname you've been given which means anytime someone /whois'es you or you join a channel that will be displayed instead of your real hostname. It's a simple (but effective) security measure to prevent people from harvesting active addresses from IRC and trying to exploit them.
