IP specified that datagram can arrive in a different order than they were sent. If a fragment from one datagram arrives at a destination before all the segments from a previous datagram arrive, how does the destination know to which datagram the fragments belong?
Tore solve the problem of reassembling the fragments which arrive out of order a unique identification number is put in the IDENTIFICATION field of all outgoing datagram. While a router fragments a datagram therefore the router copies the identification number in each segment. A receiver employs IP source address and the identification number in an incoming fragment to find out the datagram to that the fragment belongs. In addition the fragment OFFSET field informs a receiver how to order fragments inside a specified datagram.