Which field is used to identify which packet a fragment belongs to in an IPv4 header?

In an IPv4 header, the Identification field plays a crucial role in identifying which packet a fragment belongs to. When a large packet is transmitted over a network, it may be fragmented into smaller packets to accommodate the maximum transmission unit (MTU) limitations of the network. Each fragment of the original packet contains a copy of the Identification value, which is unique to the original datagram.

Using this Identification value, the receiving end can correctly reassemble the received fragments back into the original packet. This is essential for ensuring that all parts of the packet are recognized as belonging to the same original transmission, even if they arrive out of order or at different times.

The other fields in the IPv4 header serve different purposes: Flags are used to control fragmentation, Fragment Offset indicates the position of a fragment within the original packet, and Protocol specifies the higher-layer protocol used in the datagram. While these fields assist in the fragmentation and reassembly process, they do not serve the primary purpose of identifying the packet itself. Thus, the Identification field is the key element for linking fragments back to their respective original packets.