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GATE CSE 2022 Q22: Number of subnets in an enterprise network with three routers

Question

Consider an enterprise network with two Ethernet segments, a web server and a firewall, connected via three routers as shown below.

A firewall connected to the Internet, to a top router and to a left router; the top router connects to the web server and to a right router; the left and right routers each connect to an Ethernet segment with several computers
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What is the number of subnets inside the enterprise network?

Try it: multiple choice: exactly one correct option

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Wrong answer costs 1/3 mark in GATE.
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Answer: C (official GATE 2022 final answer key)

Concept used: Routers separate subnets: each router interface (each link between routers/firewall or to a LAN) is its own subnet.

First step: Count every distinct link segment bounded by router or firewall interfaces inside the enterprise, including the two Ethernet LANs and the web server link.

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Source: official GATE 2022 CS question paper and final answer key, organised by IIT Kharagpur. Question paper · Answer key

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