Identification of computers located behind an address translation server

In one embodiment, a Network Address Translation (NAT) server performs address translation services for client computers in a private computer network. A traffic monitor computer outside the private computer network may receive network data processed by the NAT server. The traffic monitor computer may parse network data to obtain application layer information, from which the traffic monitor computer may obtain identification information of the client computer that originally sent the network data. The identification information may include the private IP address of the client computer. In one embodiment, the traffic monitor computer obtains the private IP address of the client computer by examining trace information appended to an e-mail by a mail client running in the client computer.