short form for Maximum Transmission Unit is the size of each packet received by the Ethernet card. The value of MTU for all Ethernet devices by default is set to 1500. Though you can change the value by passing the necessary option to the ifconfig command. Setting this to a higher value could hazard packet fragmentation or buffer overflows. Do compare the MTU value of your Ethernet device and the loopback device and see if they are same or different. Usually, the loopback device will have a larger packet length.