GATE CS 2018 Q54 — Fragmentation and IP Addressing, IPv4, CIDR

To find the fragmentation offset of the third fragment, we analyze the payload distribution across the fragments:

1. Total IP Payload:
   $$\text{Total Payload} = \text{Total Packet Length} - \text{IP Header Length} = 4500 - 20 = 4480 \text{ bytes}$$ Note: The 40-byte TCP header is treated as part of the IP payload.
2. Maximum Payload per Fragment:
   $$\text{Max Payload} = \text{MTU} - \text{IP Header Length} = 600 - 20 = 580 \text{ bytes}$$
3. 8-Byte Alignment:
   The payload size in all intermediate fragments must be a multiple of $8$ bytes. $$\text{Fragment Payload Size} = \left\lfloor \frac{580}{8} \right\rfloor \times 8 = 72 \times 8 = 576 \text{ bytes}$$
4. Fragment Offsets:
   • Fragment 1: Carries bytes $0$ to $575$.
     $$\text{Offset} = \frac{0}{8} = 0$$
   • Fragment 2: Carries bytes $576$ to $1151$.
     $$\text{Offset} = \frac{576}{8} = 72$$
   • Fragment 3: Carries bytes starting from $1152$.
     $$\text{Offset} = \frac{1152}{8} = 144$$

Thus, the fragmentation offset value stored in the third fragment is 144.