By default, MTU is 1500 bytes. OK, the next question is how large is a byte? Technically, a byte equals 8 bits, but then you'll wonder how large a bit is, etc., and we'll. Maximum Transmission Unit (MTU) is the largest packet size your network can send without fragmenting the data. Setting the wrong MTU can cause slow connections, dropped packets, and application timeouts. We won't get into the details of these packets other than the fact that MTU determines their largest size.