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With the prevalence of Network Address Translation (NAT), identifying a number of Internet users becomes a challenging task because many users share the same public IP address. This paper proposes a passive technique for estimating a number of Internet hosts sharing the same IP address, i.e., NATted hosts. Previous work by Bellovin counted NATted hosts by observing a sequence of IPID fields in IP header. This technique only works on some operating systems with a global counter for the IPID sequence (e.g., Windows). Other operating systems that implement the IPID sequence on a per-flow or a random basis are not detected.
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