Question

Consider routing table of an organization's router shown below:\[\begin{array}{|c|c|c|} \hline \textbf{Subnet Number} & \textbf{Subnet Mask} & \textbf{Next Hop} \\ \hline \text{12.20.164.0} & \text{255.255.252.0} & \text{R1} \\ \hline \text{12.20.170.0} & \text{255.255.254.0} & \text{R2} \\ \hline \text{12.20.168.0} & \text{255.255.254.0} & \text{Interface 0} \\ \hline \text{12.20.166.0} & \text{255.255.254.0} & \text{Interface 1} \\ \hline \text{default} & \text{R3} \\ \hline \end{array}\]Which of the following prefixes in CIDR notation can be collectively used to correctly aggregate all of the subnets in the routing table?

• A. 12.20.164.0/20
• B. 12.20.164.0/22
• C. 12.20.164.0/21
• D. 12.20.168.0/22

Hint:

To aggregate multiple subnets, identify the largest common prefix in the IP addresses. CIDR notation helps to combine multiple subnets into a single address block.

Answer 1

The Correct Option is B, D

The goal is to find the smallest aggregation prefix in CIDR notation that can represent all the subnets in the routing table, which means we need to combine the subnets effectively into a single block of IP addresses. Step 1: Analyze the Subnet Ranges - Subnet 1: 12.20.164.0/22 - This represents the IP range from 12.20.164.0 to 12.20.167.255. - Subnet 2: 12.20.170.0/23 - This represents the IP range from 12.20.170.0 to 12.20.171.255. - Subnet 3: 12.20.168.0/23 - This represents the IP range from 12.20.168.0 to 12.20.169.255. - Subnet 4: 12.20.166.0/23 - This represents the IP range from 12.20.166.0 to 12.20.167.255. Step 2: Determine the Aggregation To correctly aggregate these subnets, we need to find the largest prefix that can cover all the ranges. This means looking for the common part of the subnet addresses and finding a suitable CIDR notation. - Subnet 1 (12.20.164.0/22) and Subnet 4 (12.20.166.0/23) overlap, and their combined range can be represented by 12.20.164.0/21. - Subnet 3 (12.20.168.0/23) and Subnet 2 (12.20.170.0/23) are also adjacent and can be aggregated as 12.20.168.0/22. Step 3: Choose the Correct Prefixes Now we check the options: - Option (A) 12.20.164.0/20: This CIDR prefix is too large to represent the subnets correctly. It would cover more IP addresses than needed, making it an incorrect choice. - Option (B) 12.20.164.0/22: This prefix correctly represents the first three subnets (12.20.164.0/22, 12.20.168.0/23, 12.20.170.0/23). This is the correct answer as it correctly aggregates these subnets. - Option (C) 12.20.164.0/21: This would also correctly aggregate the first subnet and partially the second, but it does not fully cover the third and fourth subnets. Hence, it doesn't fully aggregate all subnets in the routing table. - Option (D) 12.20.168.0/22: This CIDR prefix correctly aggregates 12.20.168.0/23 and 12.20.170.0/23, making it another correct option. Thus, the correct answers are (B) and (D).