In IPv4, in what units is the Fragment Offset field measured?

• A. 8-byte blocks.
• B. 4-byte blocks.
• C. 2-byte blocks.
• D. 1-byte units.

Answer: (A)

In IPv4 fragmentation, the starting position of a fragment’s data within the original datagram is given by the Fragment Offset, and that value is expressed in units of 8 bytes. The field for this offset is 13 bits wide, which fits the idea of counting 8-byte blocks: 0 to 8191 blocks, or up to 65528 bytes of payload when multiplied by 8. This design keeps the offset compact while still locating each fragment correctly in the reassembly. The first fragment has an offset of zero, and each subsequent fragment’s offset increases by the size of the previous fragment’s payload (in 8-byte units). The last fragment may be shorter than a multiple of 8 bytes, which is allowed and indicates the end of the original payload.