What is the purpose of sharing the IP ID across all fragments of a single original packet?

• A. To indicate fragmentation policy
• B. To route fragments along the same path
• C. To indicate packet priority
• D. To enable reassembly of fragments

Answer: (D)

When a packet is fragmented, each fragment carries an identification value that links it to the original datagram. All fragments from the same packet share the same IP identification field, along with the same source and destination, and the protocol. This tagging lets the destination collector recognize which fragments belong together, even if they arrive out of order. The fragment offset tells where each piece fits in the original payload, and the More Fragments flag indicates if more pieces are coming (the last fragment has this flag cleared). Because of this, the receiver can buffer fragments, piece them back together in the correct order, and reconstruct the original packet. If the ID weren’t shared across fragments, the destination wouldn’t be able to tell which fragments belong together, making reassembly impossible. This utility is specific to enabling reassembly, not about routing decisions, fragmentation policy, or signaling of priority.