MTU = Maximum Transmission Unit (oops, ya already put that as the subject )

It describes the maximum size packet, in bytes data can be sent through your network. Anything packet of data bigger than the set MTU on your computer will be chopped up into another packet (aka packet fragmentation). When they start fragmenting, your network connection slows down, and you can see lag, pages not loading, taking forever to load, and a host of other issues.

You can find the optimum MTU for your computer/network by pinging a website (best to use a well known, stable website, like google.com) with a specific pink command. Rather than rewrite it all, here's a link to a website that describes the process well.

For detailed instructions on how to save the MTU onc eyou figure out the optimized MTU for your network, here's a page with details.

If you run Windows 7 you need to run and elevated command prompt. Click Start | All Programs | Accessories, right click Command Prompt and Run as administrator

In the command window paste this:
netsh interface ipv4 set subinterface "Local Area Connection" mtu=xxxx store=persistent

xxxx is the new MTU that you're changing to. Rebooting isn't necessary. Local Area Connection is a common name for the interface if you use a NIC for internet connectivity. A wireless one may be called "Wireless Network Connection". If you're not sure, the network status icon by the clock can tell you.

If you use a router, be sure to also set the MTU to match the new MTU you just set.

For doing all of this on a Mac, see these instructions.