Chapter 15 Unified Threat Management for FortiOS 5.0 : Web filter : Advanced web filter configurations : Rate URLs by Domain and IP Address
  
Rate URLs by Domain and IP Address
Enable to have the FortiGate unit request the rating of the site by URL and IP address separately, providing additional security against attempts to bypass the FortiGuard Web Filter.
If the rating determined by the domain name and the rating determined by the IP address defer the Action that is enforce will be determined by a weighting assigned to the different categories. The higher weighted category will take precedence in determining the action. This will have the side effect that sometimes the Action will be determined by the classification based on the domain name and other times it will be determined by the classification that is based on the IP address.
 
FortiGuard Web Filter ratings for IP addresses are not updated as quickly as ratings for URLs. This can sometimes cause the FortiGate unit to allow access to sites that should be blocked, or to block sites that should be allowed.
An example of how this would work would be if a URL’s rating based on the domain name indicated that it belonged in the category Lingerie and Swimsuit, which is allowed but the category assigned to the IP address was Pornography which has an action of Block, because the Pornography category has a higher weight the effective action is Block.