Széchenyi István University SoftBank Corp. RFC 2544 has defined a benchmarking methodology for network interconnect devices. RFC 5180 addressed IPv6 specificities and it also provided a technology update but excluded IPv6 transition technologies. RFC 8219 addressed IPv6 transition technologies, including stateful NAT64. However, none of them discussed how to apply RFC 4814 pseudorandom port numbers to any stateful NATxy (NAT44, NAT64, NAT66) technologies. This document discusses why using pseudorandom port numbers with stateful NATxy gateways is a difficult problem. It recommends a solution limiting the port number ranges and using two test phases (phase 1 and phase 2). It is shown how the classic performance measurement procedures (e.g. throughput, frame loss rate, latency, etc.) can be carried out. New performance metrics and measurement procedures are also defined for measuring maximum connection establishment rate, connection tear-down rate, and connection tracking table capacity.