The Identification data field is used in various transport-oriented protocols, such as in the headers of the various IP versions, IPv4 and IPv6. This 16-bit identifier field contains a number that is incremented by one for each datagram sent.
The identifier field is used to uniquely identify data packets and to assign fragments to datagrams. Using the identifier field, the destination node is able to separate fragments of different datagrams from each other and to assemble them appropriately. The allocation of the identifier numbers is done by a higher protocol and is passed as a parameter to the IP protocol.