Why Can't I access my or my customers sites by IP Address?

This is because you/they are on a shared IP address. For customers to access their site, use the IP address that was given when you created your account in WebHostManager, followed by the customer username. Example: http://100.100.100/~username/
To access your customer control panel use the same IP address followed by "/cpanel". Example: http://100.100.100/cpanel/