LAN Speed Test with iPerf3: Network Performance Benchmarking

Why Benchmark Your LAN?

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Network performance matters for:

• NAS backups — slow file transfers = waste of time
• Homelab hypervisors — VM/container live migration
• Media streaming — 4K playback without buffering
• Troubleshooting — isolate network issues from disk/CPU limits

iPerf3 = the industry standard tool to measure real LAN throughput between two machines. Unlike speedtest.net (which tests your internet), iPerf3 tests your local network.

What You Need

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• Two computers on the same LAN (server + client)
• iPerf3 installed on both machines (open source, free)
• Wired connection recommended for accurate testing

Step 1: Install iPerf3

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For Windows:

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1. Download iPerf3 from the official website: https://iperf.fr/iperf-download.php
2. Choose the Windows version (zip file)
3. Extract the zip file to your preferred folder (e.g. C:\iperf3)

For Linux (Debian/Ubuntu):

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sudo apt-get update
sudo apt-get install iperf3

For macOS (using Homebrew):

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brew install iperf3

Step 2: Set Up the iPerf3 Server

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1. Open a terminal or command prompt on the computer acting as the server.
2. Run the following command:

Windows:

cd C:\iperf3
iperf3.exe -s

Linux/macOS:

iperf3 -s

The server will now listen for incoming connections on the default port 5201.

Step 3: Run the iPerf3 Client

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On the second computer (client), run:

Basic Command:

iperf3 -c <SERVER_IP>

Replace <SERVER_IP> with the server’s IP address.

Example:

iperf3 -c 192.168.1.100

Step 4: Advanced Testing Options

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1. Change test duration (default is 10 seconds)

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iperf3 -c <SERVER_IP> -t 30  # 30 seconds

2. Reverse mode (server sends to client)

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iperf3 -c <SERVER_IP> -R

3. Use multiple streams (parallel testing)

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iperf3 -c <SERVER_IP> -P 4  # 4 parallel connections

4. Change buffer size

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iperf3 -c <SERVER_IP> -l 8K  # 8KB buffer

5. Use UDP instead of TCP (default is TCP)

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iperf3 -c <SERVER_IP> -u -b 100M  # UDP with 100Mbps target

Step 5: Interpreting the Results

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After the test finishes, you will see output like this:

[ ID] Interval           Transfer     Bitrate
[  4]   0.00-10.00  sec  1.10 GBytes   944 Mbits/sec                  sender
[  4]   0.00-10.00  sec  1.10 GBytes   944 Mbits/sec                  receiver

• Transfer: Total amount of data transferred during the test
• Bitrate: Transfer speed in megabits per second (Mbps)

Tips and Troubleshooting

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1. Firewall: Make sure port 5201 is open on the server firewall.

2. Inconsistent results:

   • Close other applications using the network.
   • Run the test multiple times.

3. Error “connection refused”:

   • Ensure the iPerf3 server is running.
   • Double-check the server IP address.

4. Lower-than-expected speeds:

   • Check your network cables (use CAT5e or better).
   • Confirm that all devices support the expected LAN speeds (e.g. 1Gbps).

Example Automation Script (Linux/macOS)

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Create a file called network_test.sh with the following content:

#!/bin/bash
SERVER_IP="192.168.1.100"
DURATION=20
PARALLEL=4

echo "Running TCP test..."
iperf3 -c $SERVER_IP -t $DURATION -P $PARALLEL

echo -e "\nRunning UDP test..."
iperf3 -c $SERVER_IP -t $DURATION -u -b 100M

Run the script:

chmod +x network_test.sh
./network_test.sh

Conclusion

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With iPerf3, you can accurately measure your LAN performance. This tool is useful for:

• Verifying network speed
• Troubleshooting network issues
• Testing before and after network configuration changes

For best results, run the test multiple times under different network conditions (peak vs. off-peak hours).

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