Found this paper on the net: Web Sites Sharing IP Addresses: Prevalence and Significance. It’s an interesting read as I’ve always thought that blocking via just IP addresses would never work.
It’s also cool to note that the number one IP address with the most websites hosted is 18.104.22.168, a Register.com “Holding the domain name for future use” site.
Abstract: 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.
Granted it’s out of date having been written in 2002 but it’s an interesting read.