The IPID is the Identification field of the IP protocol packet header which a;long with the source address uniquely identifies the packet. The IPID is used to reassemble the packet should it be fragmented. Every fragement of the packet will then have the same IPID for the destination device identify them all to be a simgle packet.

The IPID is primarily used in the Idle Port Scanning by a hacker where he spoofs a victims IP address to then use it to scan a target network or system. Various Operating Systems generate these IPIDs in different ways.