An IP Datagram travels over different networks, each of which may have different MTU (Maximum Transmit Unit ) , which is the largest IP datagram that can be carried by a network. MTU - - maximum size of an unit which can be transmitted in a network and it depends on the network and media and how they be allowed to transmitted. also encapsulation....etc. It happens when the MTU values of hosts and routers are different from each other; When we have two different type of network/media connected with router which have different MTU in such case router broke down packet to appropriate size regarding network/media type payload exceed the Maximum Transmission Unit in the interface. so the payload has to be chopped and put them into IP fragmented packets. they are: -reassembled at destination -each fragment is independent datagram - could be routed in a different direction ...
