GATE 2018 CS – Question 64

Computer Networks · IPv4: Addressing, CIDR, Fragmentation and NAT · 2 marks · Numerical answer

Consider an IP packet with a length of 4,500 bytes that includes a 20-byte IPv4 header and a 40-byte TCP header. The packet is forwarded to an IPv4 router that supports a Maximum Transmission Unit (MTU) of 600 bytes. Assume that the length of the IP header in all the outgoing fragments of this packet is 20 bytes. Assume that the fragmentation offset value stored in the first fragment is 0.

The fragmentation offset value stored in the third fragment is ________.

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Correct answer: 144

Explanation

Each fragment can carry $600-20=580$ data bytes, and the largest multiple of 8 not above that is 576. The first two fragments carry $576\times2=1152$ bytes, so the third fragment starts at byte 1152, which is an offset of $\frac{1152}{8}=144$.