Which field in the IPv4 header indicates the position of a fragment within an original, unfragmented IP packet?

The field in the IPv4 header that indicates the position of a fragment within the original, unfragmented IP packet is the Fragment Offset. This field's purpose is to specify the exact location of the fragment in relation to the beginning of the original packet.

When an IP packet needs to be fragmented, the Fragment Offset helps the receiving device reassemble the fragments in the correct order. It indicates the number of 8-byte blocks from the start of the original packet to the start of the fragment. This means that the offset value tells the receiving system how far into the original data stream the fragment begins, which is crucial for properly reconstructing the original packet.

Other fields associated with fragmentation, such as the Fragment Identifier, are important for ensuring that all fragments belong to the same original packet, but they do not provide information about the position of each fragment. The Fragment Length is not an actual field in the IPv4 header; instead, there are fields for Total Length and Fragment Offset. Fragment Sequence is also not a defined term in this context. Understanding these specific roles helps in grasping how IP fragmentation and reassembly work in networking.