---
title: "Testing Network Performance with iperf"
source: "https://docs.nexcess.com/hosting/performance/server-optimization/testing-network-performance-with-iperf/"
description: "iperf is a network troubleshooting tool. It measures bandwidth, delay, jitter, and packet loss between two points, helping diagnose network performance."
vertical: "Hosting"
date: "2025-05-27"
last_modified: "2026-07-30"
---

# Testing Network Performance with iperf

`iperf` is an incredibly useful command-line tool for troubleshooting and assessing network performance. It helps you understand the capacity and quality of your network connection by providing metrics such as bandwidth, delay, jitter, and packet loss. This can be invaluable for diagnosing slow website loading, sluggish file transfers, or other network-related issues.

**Important Note:** When interpreting `iperf` results, remember that data transferred is typically expressed in **megabytes (MBytes)**, while network speed (bandwidth) is expressed in **megabits (Mbits)**. There are 8 bits in 1 byte, so 100 Mbits/sec is equal to 12.5 MBytes/sec.

##### Understanding iperf tests

`iperf` primarily tests maximum TCP bandwidth at Layer 4 (the Transport Layer) of the network model. Most common internet applications, like HTTP (web browsing), FTP (file transfer), and SMTP (email), rely on TCP. TCP is a “connection-oriented” protocol, meaning it establishes a reliable connection and acknowledges received messages. `iperf` can also test UDP (User Datagram Protocol) performance, which is “connectionless” and doesn’t guarantee delivery, by using the `-u` flag.

##### Installing iperf

`iperf` needs to be installed on both computers or servers that you intend to test the network connection between. Here are common installation commands for various Linux distributions:

- **On CentOS/RHEL:**

```
`yum install iperf`
```

- **On Ubuntu/Debian:**

```
`apt-get install iperf`
```

- **On Arch Linux:**

```
`pacman -S iperf`
```

##### Using iperf for network testing

`iperf` requires one machine to act as a **server** and the other as a **client**. The client connects to the server to perform the speed test.

##### Step 1: Start iperf in server mode

Run the following command on the server (the machine whose network speed you want to test from the client’s perspective):

```
iperf -s

```

**Expected Server Output (when listening):**

```
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
```

The server will now be listening for incoming connections on TCP port 5001 (by default).

##### Step 2: Run iperf in client mode

From the other server or computer (the client), run the following command, replacing `123.123.123.123` with the actual IP address or hostname of your `iperf` server:

```
iperf -c 123.123.123.123

```

This command will initiate a 10-second TCP bandwidth test from the client to the server.

**Expected Client Output (during/after test):**

```
------------------------------------------------------------
Client connecting to 123.123.123.123, TCP port 5001
TCP window size: 85.0 KByte (default)
------------------------------------------------------------
[  3] local 10.30.6.172 port 56177 connected with 123.123.123.123 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec   113 MBytes  94.8 Mbits/sec

```

**Expected Server Output (after test completes):**

Once the test completes on the client, the server will also display its results for that connection:

```
[  5] local 123.123.123.123 port 5001 connected with 10.30.6.172 port 56177
[ ID] Interval       Transfer     Bandwidth
[  5]  0.0-10.1 sec    113 MBytes  94.1 Mbits/sec

```

##### Other Useful iperf Flags:

- **-u (UDP Test):** To test UDP bandwidth instead of TCP: 
    - Server: `iperf -s -u`
    - Client: `iperf -c <server_ip> -u`
- **-d (Bidirectional Test):** To perform a simultaneous bidirectional test (client sends to server, and server sends to client): 
    - Server: `iperf -s`
    - Client: `iperf -c <server_ip> -d`

### Conclusion

`iperf` is a fundamental tool for network diagnostics, providing clear, quantifiable data on your network’s performance. By running tests between your servers or between your local machine and a server, you can quickly identify network bottlenecks, verify bandwidth, and ensure your applications have the connectivity they need. For any complex network issues or assistance with `iperf` results, Liquid Web’s [Support team](https://www.liquidweb.com/support) is always ready to help!