NAT (Network Address Translation) is widely used as a technique that shares a single IP address in several PCs, and is widely used for alleviating the IPv4 address exhaustion and as a security solution. However, when a backbone network has access control function for PCs based on their IP addresses, it can not identify the PCs behind a NAT router since their original IP addresses are hidden by the NAT router. In this research, to solve this problem, we focus on www which is the most popular service of the Internet and propose an identification method of PCs behind a NAT router with proxy authentication. In this method, we modified the proxy server so that it is able to identify PCs by using the field called realm in the authentication header and allocating it uniquely to PCs. © 2011 IEEE.