IPerf is a tool that lets you measure and tune network performance. IPerf can be run in client or server mode. Generally, client mode is used.

iPerf requires you to know of a reliable public server with which to 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: here, (visible only in Client mode), enter the target iPerf server address.

Protocol TCP/UDP: here, you specify the protocol to be used for testing.

Port: here, enter the port to be used for testing (Default: 5201).

Type: here, specify under what conditions testing will stop:

• Time-limited - choosing this will make testing stop after
  a certain time period.
• Buffer-limited - choosing this makes testing stop after
  a certain amount
  of data has been transferred.

Time Limit/Bytes limit: here, enter the units used for the limit type 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 were far from accurate. Regardless, iPerf can still be a useful tool for tasks such as intranet VPN performance testing and tuning.