What is the purpose of the Identification field in the IPv4 header?

• A. To identify the destination network
• B. To help assemble fragments of the original packet by matching fragments
• C. To indicate the protocol of the payload
• D. To carry the source port

Answer: (B)

When an IPv4 packet is larger than what a network can carry, it may be split into smaller fragments that all belong to the same original datagram. To reassemble them correctly, the receiver needs a way to recognize which fragments go together. The Identification field provides a unique 16-bit value that is the same for all fragments of a single datagram and different for fragments from other datagrams. This value, used alongside the source and destination addresses and the protocol, lets the destination host collect all relevant fragments and reconstruct the original packet using the Fragment Offset to place each piece in order. If a packet isn’t fragmented, this field isn’t needed for reassembly, though it still carries a value. The other options describe different header information: the destination network is determined by the destination address, the protocol field indicates the transport layer protocol, and the source port is part of the transport header, not the IP header.