rhn_register unable to find Red Hat Network

Hi,

I installed Red Hat Enterprise Linux 5.3, and want to register to Red Hat Network, but when I launch rhn_register and go forward to insert credentials, it show an error (see attached picture) that say which it cannot reach xmlrpc.rhn.redhat.com; note that internet connection is active.

There are any setting to do before?
Thanks in advance for Your help.
Cordially.
Mauro Rosati
Infordata S.p.A.