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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 [1] 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. The proposed technique overcomes this limitation by observing patterns of the TCP sequence number and the TCP source port, in addition to the IPID sequence. Our technique demonstrates more accurate estimate than the previous work in controlled experiments. Moreover, applying our technique on a collection of longitudinal traffic traces measured at a trans-Pacific link in 2001–2010, we find that the percentage of the NATted hosts is stably less than 2% over years. [ABSTRACT FROM PUBLISHER]