Does the IP address you're using server more than one site? If so, how will it know which specific site you're trying to view without a domain name?
The only way it would work is if you edited your hosts file (/etc/hosts or C:\Windows\system32\drivers\etc\hosts (mac I'm not sure)) and alias that domain to that specific IP. Once again though, you'd have to use the domain name.
That's all I can really think of doing. Why are you accessing the site(s) via IP instead of their domain names?