If you have an .htaccess file, thats controlling it.
Default would be to drop anything non-local, I would suspect. What you're receiving is a message because you do not allow directory listings externally. If you put an index.[html|php whatever] on there, it should still display that page.
You're options are to modify you're httpd.conf file to enable directory listings, or to add a rule in a .htaccess. This is unreliable unless you're provided IP is static. You could open this up completely to allow everyone directory listings, but I wouldn't do that.
Do you have a reason to be using the ip address instead of a loopback address?
Oh yes, I forgot to mention.
This isn't a PHP problem, so I'm moving it to other server side issues.