PAT (overloading) divides the available ports per global IP address into three ranges: 0-511, 512-1023, and 1024-65535. PAT assigns a unique source port for each UDP or TCP session. It attempts to assign the same port value of the original request, but if the original source port has already been used, it starts scanning from the beginning of the particular port range to find the first available port and assigns it to the conversation. There is an exception for 12.2S code base. 12.2S code base uses different port logic, and there is no port reservation.