Maximum Transmission Unit (MTU)
Each hardware method specification adds the definition of the maximum size of the frame data area, which is known the Maximum Transmission Unit (MTU). Any datagram packet encapsulated in a hardware frame have to be smaller than the MTU for that hardware.
MTU AND HETEROGENEOUS NETWORKS:
An Internet can have networks with different MTUs as given in the figure below. Consider downstream network has smaller MTU than local network.