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We call end-hosts behind network address translator (NAT) gateways or firewalls guarded hosts, and otherwise open hosts. In this paper, we empirically measure the prevalence of guarded hosts in two popular peer-to-peer file sharing systems, eDonkey and Gnutella, and study the characteristics of their shared files. By performing passive and active probes, we found that about 25-36% of eDonkey and Gnutella users reside on guarded hosts and that the ratio of files shared by guarded hosts is also non-trivial. When discounting guarded hosts, we found that a popular file's availability, i.e., the number of copies available for download, decreases by 25-30%. Our measurement study testifies to the significant impact guarded hosts may have on the performance of current peer-to-peer file sharing systems, and points to a need to consider their presence when designing next generation peer-to-peer systems.