AdventuresInVista
Hints, Tips & Tricks for Microsoft Windows Vista

Tuesday Tweak - Change your MTU
11 March 2008 in Tuesday Tweaks | Comments enabled

MTU (or ‘Maximum Transfer Unit’) is used by all networks to limit the amount of bytes in a packet. Normally, this is set to 1500 bytes… but there are some networks that just dare to be arkward.

My service provider - AOL - is one such case.

AOL (in the UK at least) require an MTU setting of 1400. If you use the AOL software, this stuff is handled for you. But if, like me, you’ve set everything up manually, you’ll need to change the MTU setting on both your router and all your computers.

I thought I’d done this - web browsing and email have been working fine on the various PCs for ages. With the arrival of the Mac Mini (see AdventuresInMacOS), I’ve reorganized everything. Email has moved to my Vista machine. Everything seemed normal, except I couldn’t send mail…

I tried a few things, and then remembered about the MTU. I needed to check the settings. To do that you need to use Command Prompt with elevated privileges…

• Click the “Start” logo button
• Click “All Programs”
• Click “Accessories”
• Right click “Command Prompt” and choose “Run as administrator”

At this point you’ll be presented with User Account Control, so just click continue or enter your Administrator password if necessary.

• Once in command prompt, type..
  netsh interface ipv4 show subinterfaces
  … and press ENTER. Your command window should look similar to the following…

The interface you’ll be interested in depends on your computer setup. If you’re connected to your network via a cable, like me, then you want to know he “Local Area Connection”. If you using wireless connections, look for “Wireless Network Connection”.

The MTU setting is the first column, and as you can see my Vista PC’s Local Area Connection is set to 1500. If you need to change your MTU setting, use the following command…

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1400 store=persistent

Remember to substitute “Local Area Connection” with your connection name (with the quotes), and “1400″ with the MTU you require (without the quotes!)

You should receive a message that simply says “Ok” - and you’re done! You’ll need to restart your computer for the changes to take effect though.

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