In a fragment train, which statement best describes fragment sizes?

• A. All fragments have the same size except the last, and all sizes are less than or equal to the MTU.
• B. All fragments have equal size to the MTU
• C. The last fragment is larger than MTU
• D. Fragment sizes are random

Answer: (A)

When an IP datagram is fragmented, the payload is broken into pieces that fit within the MTU. To ease reassembly, the fragments—except for the final one—carry the same amount of data. The final fragment holds whatever data remains and may be smaller. None of the fragments can exceed the MTU, so every fragment’s size is at most the MTU. If the original datagram length happens to be an exact multiple of the chosen fragment payload size, the last fragment will match the others, but in general it can be smaller. This is why the description that fits best is that all fragments have the same size except the last, and all sizes are less than or equal to the MTU.