If your broadband Internet connection (cable or DSL) is slower than expected, your Internet Service Provider (ISP) may suggest that you change the maximum transmission unit (MTU) as a troubleshooting step.
Important: Changing the MTU is recommended only when done under the advice of your ISP or an experienced troubleshooter. Improper configuration can result in a loss of connectivity.
Steps for Ethernet in Mac OS X 10.5.x
- From the Apple menu, choose System Preferences.
- Click Network.
- In the left sidebar, select "Built-in Ethernet 1".
- If you see a lock icon in the bottom left corner, click the lock icon and enter your administrator password.
- Click the "Advanced..." button.
- Click the Ethernet tab.
- From the Configure: pop-up menu, choose "Manually".
- From the MTU pop-up menu, choose "Custom".
- Enter your desired setting (72 to 9000). Some ISPs may have a specific suggested MTU value. If you're changing it without a specific recommendation, make incremental changes (such as 1500 to 1400 to 1300, and so forth).
- Click OK, then click Apply to save your changes.
Important: Changes made to your Ethernet MTU settings will be persistent, meaning the new values will still be used after you restart your computer.
Steps for AirPort in Mac OS X 10.5.x
- From the Apple menu, choose System Preferences.
- Click Network.
- From the Show pop-up menu, choose AirPort.
- Click the Ethernet tab.
- From the Configure pop-up menu, choose Manually.
- From the MTU pop-up menu, choose Custom.
- In the field to the right of Custom, enter your new setting. Some ISPs may have a specific suggested MTU value. If you're changing it without a specific recommendation, make incremental changes (such as 1500 to 1400 to 1300, and so forth).
Important: Changes made to your Ethernet MTU settings will be persistent, meaning the new values will still be used after you restart your computer.
For Mac OS X 10.4.x
Note: For these steps below (Mac OS X 10.4.x), you must be logged into Mac OS X as a user who has administrator privileges. You can see which users are administrators in the Accounts pane of System Preferences.
- Open the Terminal, which is located in the Applications folder, inside the Utilities folder.
- At the command prompt, type the following command. In place of "<value>" enter the number for the MTU value you wish to set.
$ sudo ifconfig en1 mtu
For example, this would set the MTU value for the AirPort interface to 1400:$ sudo ifconfig en1 mtu 1400
- You will be prompted to enter your password, and then the command will be completed. You can verify the setting of the MTU value with this command:
$ ifconfig en1
This will return all current information about your "en1" (AirPort) network interface as seen at the end of the second line of text in this image:

Note: The AirPort MTU setting made via Terminal is not persistent, meaning that you will need to repeat these steps each time you restart your computer.