GATE CSE 2022 Q55: CIDR prefixes that aggregate a router's subnets

Consider routing table of an organization's router shown below:

Which of the following prefixes in CIDR notation can be collectively used to correctly aggregate all of the subnets in the routing table?

Try it: multiple select: one or more correct options

Select all correct options. No negative marking, no partial marks.

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Answer: B, D (official GATE 2022 final answer key)

Concept used: Write the third octet ranges covered by each subnet and find valid prefixes (network address aligned to the mask) that cover them.

First step: 164.0/22 covers 164–167 (which includes 166.0/23); 168.0/23 and 170.0/23 together cover 168–171.

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