GATE CS 2022 Set 1 — Question 55
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Computer Networks → Network Layer: Addressing → Subnetting & CIDR
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Question
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?
| Subnet Number | Subnet Mask | Next Hop |
|---|---|---|
| 12.20.164.0 | 255.255.252.0 | R1 |
| 12.20.170.0 | 255.255.254.0 | R2 |
| 12.20.168.0 | 255.255.254.0 | Interface 0 |
| 12.20.166.0 | 255.255.254.0 | Interface 1 |
| default | R3 |
Correct answer
(B) 12.20.164.0/22; (D) 12.20.168.0/22
Solution
To aggregate the subnets, we first determine the range of IP addresses covered by each entry in the routing table:
1. (Mask ): Range to .
2. (Mask ): Range to .
3. (Mask ): Range to .
4. (Mask ): Range to (this is a subset of the first range).
The total range to be covered is to . - The range is exactly covered by the prefix (Option B).
- The range (combining and ) is exactly covered by the prefix (Option D).
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