MTU: Maximum Transfer Unit. It is calculated in bytes. For example, commonly for ethernet devices the MTU is set to 1500.

MTU is set in the device configuration settings for Ethernet, Token ring etc.

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In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The mtu may be required by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-point serial links. A higher MTU brings higher bandwidth efficiency. However large packets can block up a slow interface for some time, increasing the lag on other packets. For example a 1500 byte packet, the largest allowed on an Ethernet, will block up a 14.4k modem for about one second.

Source: http://en.wikipedia.org/wiki/Maximum_transmission_unit
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