If authentication is enabled on the web ui, then a cookie is used to keep track of the authentication information. There is normally a domain associated with the cookie. Since ip address doesn't have any domain , the cookie will not be sent by the browser while making http calls with ip address as the destination server.
This will break browsing files system via web ui , if authentication is enabled.
Browsing FileSystem via web ui, should use datanode's fqdn instead of ip address.