IPerf is a tool which allows you to measure and tune network performance.
IPerf can be run in client mode or server mode. Generally, Client mode is most often 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
Some don't work, so if you receive an error when testing with a non-functioning server, try a different iPerf server. You can also try unofficial iPerf servers, but their location and availability may change often. Once you've configured the Server address, protocol and port, you can start your tests. You'll probably only use iPerf's 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 TCP/IP layer 4 protocol to be used for testing.
Port: Here you enter the port to be used for testing (Default: 5201).
Type: Select testing should stop after a specific amount of time (Time-limited), or after a certain amount of of data has been transferred (Buffer-limited).
Time/Bytes limit - Unit used for the limit, as specified in Type.
Unfortunately, there are currently few reachable iPerf servers. Also, it's important to use an iPerf server with geographical proximity to get accurate results. The above example was run on a 200Mb downstream / 12MB upstream connection. However, as you can see, the results are far from accurate. Nevertheless, the IPerf tool can still be useful for intranet VPN performance testing and tuning.