Where did you read that? You need one device as a client, one as a server. So two devices in total. Running it to 127.0.0.1 would be meaningless. For example:
Code:
[xxxx@xxxxxxxxx ~]$ iperf3 -c 127.0.0.1
Connecting to host 127.0.0.1, port 5201
[ 5] local 127.0.0.1 port 53964 connected to 127.0.0.1 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 6.07 GBytes 52.2 Gbits/sec 0 1.75 MBytes
[ 5] 1.00-2.00 sec 6.19 GBytes 53.1 Gbits/sec 0 2.12 MBytes
[ 5] 2.00-3.00 sec 6.05 GBytes 51.9 Gbits/sec 0 2.25 MBytes
[ 5] 3.00-4.00 sec 6.13 GBytes 52.7 Gbits/sec 0 2.69 MBytes
[ 5] 4.00-5.00 sec 6.10 GBytes 52.4 Gbits/sec 0 3.68 MBytes
[ 5] 5.00-6.00 sec 6.21 GBytes 53.3 Gbits/sec 0 3.68 MBytes
[ 5] 6.00-7.00 sec 5.89 GBytes 50.6 Gbits/sec 0 3.68 MBytes
[ 5] 7.00-8.00 sec 6.05 GBytes 51.9 Gbits/sec 0 3.68 MBytes
[ 5] 8.00-9.00 sec 6.03 GBytes 51.8 Gbits/sec 0 3.68 MBytes
[ 5] 9.00-10.00 sec 6.10 GBytes 52.4 Gbits/sec 0 3.68 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 60.8 GBytes 52.2 Gbits/sec 0 sender
[ 5] 0.00-10.04 sec 60.8 GBytes 52.0 Gbits/sec receiver
iperf Done.
That device is on a 10Gbps local network with a 1Gbps internet connection.
If you're wanting to test wireless performance then have one device hard wired, the other on wireless. It doesn't matter which way around you have it. By default the data will go from client to server so if the wired device is the server you'd be testing the upload of the wireless device. Run with the -R flag and it'll run in reverse so the server will send to the client. Downstream in this instance.