[via whump.com] The paper  discusses the problems of webservers hosting multiple sites to be reached by the same IP. This might become a problem, where one wants to block a specific site from this server, but still keeping the others.
Well, my first thought was, that one could use some HTTP_HOST filtering instead, but maybe I should read further… Have a look at a sample page with blocked sites .
More than 87% of active domain names are found to share their IP addresses (i.e. their web servers) with one or more additional domains, and more than two third of active domain names share their addresses with fifty or more additional domains. While this IP sharing is typically transparent to ordinary users, it causes complications for those who seek to filter the Internet, restrict users\’ ability to access certain controversial content on the basis of the IP address used to host that content. With so many sites sharing IP addresses, IP-based filtering efforts are bound to produce \”overblocking\” — accidental and often unanticipated denial of access to web sites that abide by the stated filtering rules.