The reason IP won't work is that the IP is shared among all users on the server, this means all domains (URL) that is used on any account have the same IP, so using the IP alone the server have no idea what content you expect and just responds with the defaultpage.
This is normal behavior for shared hosting, however on our
premium hosting service you can pay extra to have a dedicated IP (some plans have it included for free) if you wish to be able to use IP alone.