Computer Networks · M2 · read

IPv4 fragmentation · optional extension

IP packet details · supporting practice

Unit 4 support · header comparison; fragmentation is additional practice

Optional worked extension. This calculation format was not found in the supplied December 2025 final or February 2026 re-exam. The numbers and practice marks are authored. Study the core concept and actual-paper questions first; this is extra explanation, not a TEE prediction.

Definition. IPv4 fragmentation divides a packet that exceeds an outgoing link’s MTU into smaller fragments, when fragmentation is permitted.

Simple meaning. MTU se packet bada hai toh allowed case mein fragments; offset original payload mein position batata hai.

Understand it step by step

  1. This is supporting practice, not an explicitly listed numerical in the supplied current syllabus or these two papers. Finish the confirmed core first.
  2. Assume an originally unfragmented packet and a fixed header H repeated per fragment. The link MTU is the largest whole IP packet it can carry.
  3. Every non-final fragment’s payload must be a multiple of 8 bytes. Its payload capacity is 8×floor((MTU−H)/8).
  4. Fragment Offset is the number of preceding original payload bytes divided by 8; it is not a byte count directly. MF=1 means more fragments follow, while the final fragment has MF=0.
  5. If DF is set, a router cannot fragment the IPv4 packet; IPv6 routers do not fragment packets either. Reassembly is at the destination.

Practice question. What is the unit of IPv4 fragment offset and what does MF indicate? [3 practice marks]

Answer

Fragment Offset is measured in units of 8 payload bytes. MF=1 says more fragments of the original packet follow; MF=0 marks the last fragment. Each fragment carries its own IP header.

Yaad rakho: Offset ×8 = starting payload byte.