How does IPv4 fragmentation occur and what field tracks the offset?

• A. Packets larger than the MTU can be fragmented by routers; Fragment Offset field tracks the position of a fragment's data.
• B. Packets smaller than the MTU are fragmented; Fragment Length field tracks offset.
• C. Packets larger than MTU are not fragmented; Fragment Offset tracks time-to-live.
• D. Fragmentation only happens in IPv6; Fragment Offset is not used.

Answer: (A)

Fragmentation happens when a router on the path can't forward a datagram because its size exceeds the MTU of the next link. To get the data through, the router splits the original packet into smaller fragments, each with its own IPv4 header but carrying a portion of the payload. All fragments share the same Identification value so they can be reassembled at the destination, and the position of each fragment’s data within the original payload is indicated by the Fragment Offset field. That offset is measured in 8-byte units, so the first fragment starts at offset zero and subsequent fragments report the starting position of their data in 8-byte chunks. The final fragment has the More Fragments flag cleared to signal the end of the datagram. For context, you can think of how Ethernet MTUs around 1500 bytes influence how much payload each fragment can carry after accounting for the header; reassembly uses the Identification, Fragment Offset, and MF flag to reconstruct the original datagram.