Masquerading exploits a characteristic of TCP/IP data transmission, in that source and destination port numbers are used in addition to the source and destination addresses. When a router receives a data packet for transmission, it notes the IP address and the port of the sender in an internal table. The packet is then given the IP address of the router and an arbitrary new port number. This new port number is also entered in the table, and the packet is forwarded with its new IP address and port number.