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.
The response to this packet is now returned to the router’s IP address together with the sender port number. The router can now forward the response to the original sender by using the entry in the internal table.
