Reserved ports allows an operator to configure a small number of ports to be reserved for designated applications should a port range block be exhausted. Such a scenario may occur when a subscriber is unwittingly subjected to a virus or engaged in extreme cases of P2P file transfers. In these situations, instead of blocking all new mappings indiscriminately, the 7750 SR NAT application allows operators to nominate a number of reserved ports and then assign a 7750 SR forwarding class as containing high priority traffic for the NAT application. Whenever traffic reaches the NAT application which matches a priority session forwarding class, reserved ports are consumed to improve the chances of success. Priority sessions could be used by the operator for services such as DNS, web portal, e-mail, VoIP, and so on, to allow these applications even when a subscriber exhausted their ports.