What is MTU Size and What Should I Set it to?

If you’re connected to a network, you already know about routers. Routers allow you to connect to your network wirelessly through Wi-Fi, or through a wired Ethernet connection. Many people, particularly those who aren’t experienced in setting up networks, simply plug up their router and immediately go online. While this may certainly work, this might not be ideal. You may notice decreased speeds online, lags, or you may even become completely disconnected. While this may be chalked up to problems with the ISP or equipment itself, some problems may be due to the settings of the router.

One of these settings is the MTU. Changing this setting on your equipment can optimize your internet speeds and connectivity. But to do so effectively, you must first understand what MTU is and how to find the right settings for your network.

What Exactly Is MTU?

MTU stands for Maximum Transmission Unit. This is just one of many settings that you may have to adjust to fully optimize your router. The MTU size is a setting that determines the largest packet size that can be transmitted through your system. These packets are measured in octets, or eight-bit bytes. The Transmission Control Protocol determines the maximum packet transmission size based on your MTU settings. The MTU settings can be configured through your router’s advanced configurations.

When to Adjust Your MTU Size

If you notice that your internet isn’t as fast as it should be or other performance issues, you might want to begin troubleshooting by adjusting your MTU size. The process is fairly quick and easy, and it doesn’t require any additional software or hardware installations. In fact, you can test your settings through DOS and change them through your router’s advanced configurations, as already noted.

As a rule of thumb, a larger MTU size will increase bulk protocol throughput. Fewer packets with the same amount of data are processed when the MTU size is increased. However, the tradeoff is that may lead to delays in subsequent packets, resulting in lags and minimum latency. On the flip side, a smaller MTU size will result in more overhead and acknowledgements that must be sent and received. For these reasons, you need to troubleshoot to make sure that your setting is not too high or not too low. Since every system and network is different, there are a few steps you will need to take in order to optimize your MTU size, resulting in optimum performance of your system.

Finding the Optimum MTU Size for Your Needs

Some of your wireless router settings may require you to install additional hardware or software, but this isn’t required for optimizing your MTU size. In fact, all you’ll need to do is access your computer’s command prompt and know a few important commands that can help you determine the optimum MTU size for your system.

To begin, open your command prompt to begin what is known as the ping test. One your command prompt is open, you will input the following command for Windows computers: ping [url / local server or IP address] -f -l xxxx. It is important to note that the brackets should be removed prior to inputting this command, the correct URL or IP being tested should be input, and the final digits should be the packet size value that is being tested.

If you are on a Mac computer, the command is slightly different. You will input ping [url / local server or IP address] -d -s xxxx, keeping in mind the notes stated above.

If a message is received that the packet needs to be fragmented, the test packet size can be dropped by 10 until the packet is no longer fragmented. Then, you can gradually increase the number until you find the maximum packet size that does not fragment.

Once this number is found, you’re not finished quite yet. Take your test results and add 28 to it. This final number is the optimum MTU size for your system. You can change this number within your router’s advanced configurations (consult your user’s manual or manufacturer’s website for instructions on how to access these settings) or you can go into your computer’s System Preferences menu to manually change the settings for built-in Ethernet.

Unsure of where to start testing? Some good numbers to begin testing for your test sample are 500, 600, 625, 650, 750, 1,000 and 1,500.

Once you change the value, even slightly, it’s important to test your network speed. You want to make sure that everything is working as it should be, moving at optimum speed and without lags and interruptions.

Final Thoughts and Notes About MTU Size

If you ever experience problems when connecting to the internet or using your network, don’t just assume that it’s because of a slow ISP or outdated equipment. While this very well may be the case, it’s also possible that you just need to customize your router and systems for optimum performance. This always includes performing ping tests and changing up your MTU size.

Remember, it’s important to:

  • Use the ping test to test your MTU size. You want to make sure that you find the maximum packet size that is transmitted without fragmentation.
  • Add 28 to the number you find in your ping test to get your complete MTU size.
  • Even if you perform all steps properly, it’s important to make sure that your network is still running at desired speeds and that your system isn’t experiencing any lags, interruptions, disconnections, or other issues.
  • MTU size should always be set to the same value for all computers within your network.

Have you had success finding and setting your optimum MTU size? Do you have any recommendations for those who are new to adjusting their settings? Are you facing issues when trying to find your optimum settings? Let us know in the comments!

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