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GATE 2016 CS – Question 63

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

An IP datagram of size 1000 bytes arrives at a router. The router has to forward this packet on a link whose MTU (maximum transmission unit) is 100 bytes. Assume that the size of the IP header is 20 bytes.

The number of fragments that the IP datagram will be divided into for transmission is ________.

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Correct answer: 13

Explanation

Each fragment can carry $100 - 20 = 80$ bytes of data, and 80 is a multiple of 8, which fragment offsets require. The datagram holds $1000 - 20 = 980$ bytes of data. The number of fragments is $\lceil \frac{980}{80} \rceil = \lceil 12.25 \rceil = 13$.