Several web-sites (domains) might be hosted on the same server (i.e. identical IP address).
But when accessing the web-site directly via IP, your browser doesn't tell the server which domain it actually wants. That's why it won't work with some web-sites.
Solution: Use one of the various free web-proxies, if these aren't blocked too at your location.
Choose on of these:
http://proxy.org/cgi_proxies.shtml
(BTW: Edited your thread title to something more suitable. Please choose a proper title next time!)