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GATE CSE 2020 Q48: Candidate /21 blocks from 202.61.0.0/17 for 1500 hosts

Question

An organization requires a range of IP addresses to assign one to each of its 1500 computers. The organization has approached an Internet Service Provider (ISP) for this task. The ISP uses CIDR and serves the requests from the available IP address space 202.61.0.0/17. The ISP wants to assign an address space to the organization which will minimize the number of routing entries in the ISP's router using route aggregation. Which of the following address spaces are potential candidates from which the ISP can allot any one to the organization?

I. 202.61.84.0/21

II. 202.61.104.0/21

III. 202.61.64.0/21

IV. 202.61.144.0/21

Try it: multiple choice: exactly one correct option

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Wrong answer costs 2/3 mark in GATE.
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Answer: B (official GATE 2020 final answer key)

Concept used: A valid CIDR block starts at a multiple of its size and must lie inside the ISP's own prefix.

First step: A /21 spans 8 values of the third octet, so it must start at a multiple of 8; the /17 covers third octets 0 to 127.

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Source: official GATE 2020 CS question paper and final answer key, organised by IIT Delhi. Question paper · Answer key

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