If a packet arrives with an M-bit value of ‘1’ and a fragment offset value of…

Concept — the two IPv4 header fields that place a fragment.

When a router splits an IP datagram to fit a smaller MTU, every resulting fragment carries two positional markers in its own header. The More Fragments bit (the M-bit) is a flag: M = 1 announces that at least one further fragment of the same datagram follows it, while M = 0 marks the final piece of the set. The Fragment Offset is a 13-bit field giving where this fragment’s payload sits inside the original datagram, counted in units of 8 bytes; an offset of 0 therefore means the payload begins at the very start of the original data, with nothing before it.

Applying the two markers to this packet.

1. The arriving header has M = 1, so more fragments of the same datagram are still to come and this packet cannot be the closing piece of the set.

2. Its fragment offset is 0, so its payload begins at byte 0 of the original datagram and no other fragment’s data precedes it.

3. A piece that begins the original data and is still followed by other pieces is the opening piece of the fragment set — the first fragment.

Cross-check — all four combinations of the two fields.

Each IPv4 header holds exactly one value in each of these two fields, so an arriving packet matches exactly one row of the table. The pair M = 1 with offset = 0 matches only the opening row, so the blank is filled by ‘first’.