GATE 2025 CS (CS1) – Question 57

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

Suppose a message of size 15000 bytes is transmitted from a source to a destination using IPv4 protocol via two routers as shown in the figure. Each router has a defined maximum transmission unit (MTU) as shown in the figure, including IP header. The number of fragments that will be delivered to the destination is ________ . (Answer in integer)

Source -> Router-1 (MTU=5000 bytes) -> Router-2 (MTU=3000 bytes) -> Destination.

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Correct answer: 7

Explanation

With a 20-byte header, Router-1 allows 4980 data bytes, rounded down to 4976 (a multiple of 8). So 15000 becomes 3 fragments of 4976 plus one of 72. Router-2 allows 2976 data bytes, so each 4976 splits into 2976 and 2000, giving 3·2+1 = 7 fragments.