Just for shits and giggles: 2014 MBP/10.14.6: ./iperf3 -c 127.0.0.1 Connecting to host 127.0.0.1, port 5201 [ 4] local 127.0.0.1 port 61107 connected to 127.0.0.1 port 5201 [ ID] Interval Transfer ...

Usually, address 127.0.0.1 will be assigned to a special interface on your host, the so-called loopback interface, which acts like a closed circuit.