IPerf is a tool that allows you to measure and tune network performance. IPerf can be run in client mode or server mode. Generally, client mode is used.
iPerf requires you to know of a reliable public server with which you can use it. Currently, the official IPerf webpage hosts a short list of public iPerf servers: https://iperf.fr/iperf-servers.php.
Some iPerf servers don't work, so if you receive an error when testing, try a different server. You can also try unofficial iPerf servers, but their location and availability may often change. Once you've configured the Server address, protocol and port, you can start your tests. You'll probably only use Server mode if you're testing an intranet VPN site.
Mode Server/Client: Here you set the mode of operation.
Host address: This field is only visible in Client mode. Here, you enter the target iPerf server's address.
Protocol TCP/UDP: Here you specify the protocol to be used for testing.
Port: Here, you enter the port to be used for testing (Default: 5201).
Type: Here, you specify under what conditions testing will stop:
Time Limit/Bytes limit - In this field, you enter the the units used for the limit type you selected in the Type setting.
Unfortunately, there are currently few reachable iPerf servers. In order to get accurate results, it's important to use an iPerf server with geographical proximity. The above example was run on a 200Mb/s downstream / 12Mb/s upstream connection. However the results are far from accurate. Regardless, IPerf can still be a useful tool for tasks such as intranet VPN performance testing and tuning.