./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 Bandwidth [ 4] 0.00-1.00 sec 6.00 GBytes 51.6 ...
you are studying for a network exam and you don't know THAT number?<BR><BR><BR>Anyways, that is the loopback number, and packet sent to a 127.x.x.x number goes to itself. It is used to test.