Netcraft have a somewhat vague article on their methodology on their site eg.<p>&gt;By arranging for a number of IP addresses to send packets to us near simultaneously, low level TCP&#x2F;IP characteristics can be used to work out, within an error margin, if those packets originate from the same computer, by checking for similarities in a number of TCP&#x2F;IP protocol header fields. To build up sufficient certainty that IP addresses on the same computer have been identified, many visits to the sites in the Web Server Survey are necessary, which takes place during a period of over a month.<p>&gt;Informal checking with two of the largest dedicated server companies suggests that the survey counts about 2⁄3 – 3⁄4 of the machines in their datacenters.<p><a href="http:&#x2F;&#x2F;www.netcraft.com&#x2F;internet-data-mining&#x2F;hosting-provider-server-count&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.netcraft.com&#x2F;internet-data-mining&#x2F;hosting-provide...</a>