From Netsurf: "AT&T researcher Steven Bellovin has discovered a way to count the number of computers behind an organization's firewall. More specifically, he discovered that it is possible to deduce the number of servers behind a Network Address Translator (NAT) box - usually part of the firewall - by looking at certain predictable fields in the IP packets that it generates. NATs are widely used to mask multiple servers behind a single IP address, both for security reasons and to conserve the number of globally available IP addresses. His research paper offers technical details about the method and its limitations. This discovery is generally bad news from the standpoint of security and privacy and no simple global fix is available. On the other hand, this method should let demographers get a better count of the true number of computers connected to the Net. It's a clever bit of research."

http://www.research.att.com/~smb/papers/fnat.pdf

Very good guide to Collocation

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