Which value is shared across all fragments of the same original IPv4 packet?

• A. Time to live (TTL) value
• B. Fragment offset
• C. IP ID
• D. MF bit

Answer: (C)

The key idea here is how IPv4 fragmentation is identified and reassembled. When a single IP datagram is broken into pieces, the identification value in the IP header is the same for every fragment that came from that original packet. This constant ID lets the destination know that these fragments belong together and should be reassembled into one datagram, even though they arrive separately.

Other header fields don’t stay the same across all fragments. Time to live can be decreased by each hop, so different fragments may carry different TTL values by the time they arrive. The fragment offset is different for each piece because it indicates where that fragment’s data fits within the original payload. The MF (More Fragments) bit is used to signal whether more fragments follow; it is set on all fragments except the last one. Because of these properties, the only field that reliably marks all pieces as belonging to the same original packet is the IP identification value.