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GATE CSE 2025 Session 1 Q40: Longest prefix match for 145.36.109.70

Question

A packet with the destination IP address 145.36.109.70 arrives at a router whose routing table is shown. Which interface will the packet be forwarded to?

Subnet AddressSubnet Mask (in CIDR notation)Interface
145.36.0.0/16E1
145.36.128.0/17E2
145.36.64.0/18E3
145.36.255.0/24E4
Default--E5

Try it: multiple choice: exactly one correct option

Select one option
Wrong answer costs 2/3 mark in GATE.
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Answer: A (official GATE 2025 final answer key)

Concept used: Longest prefix matching in IP forwarding.

First step: Write the third octet 109 in binary (01101101) and check which entries' prefixes it matches; pick the longest match.

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Source: official GATE 2025 Session 1 CS question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key

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