Dynamic allocation of ports at firewall

A system, apparatus, and method for dynamically allocating ports in a firewall is presented herein. During establishment of a data transfer session, such as a voice over IP call, the firewall receives signals which establish the data transfer session. The foregoing signals indicate the identity of the terminals as well as the port numbers used by the terminals. The firewall records the foregoing information. During the data transfer session, data packets for a terminal in the network of the firewall are examined for addresses and port numbers of the sender and destination. Wherein the foregoing information matches the information recorded during establishment of the data transfer session, the data packets are permitted to reach the terminal. Additionally, at the termination of the data transfer session, the record of the data transfer session is deleted, or otherwise indicated as invalid, and additional data packets received for the terminal are prevented from reaching the terminal, notwithstanding inclusion of the previously stored port numbers.