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?

Question

With respect to a TCP connection between a client and a server, which of the following is true?

Answer 1

Step 1: Convert the third octets to binary

All subnets share the first two octets (12.20). Let's examine the third octet for each:

Step 2: Identify common bits and boundaries

Looking at the binary values, we see two distinct groups forming based on their bit patterns:

Group A (164 and 166):

Both share the first 6 bits: 101001xx. This corresponds to a /22 prefix (8+8+6 bits). The network address is the lowest value: 12.20.164.0/22.

Group B (168 and 170):

Both share the first 6 bits: 101010xx. This also corresponds to a /22 prefix. The network address is: 12.20.168.0/22.

Step 3: Evaluate for a single aggregate

Can we combine Group A and Group B?

Common bits for all four: 1010xxxx.
This would be the first 4 bits of the third octet, totaling a /20 prefix (8+8+4).
However, the question often looks for the most efficient specific aggregates present in the table.

Step 4: Verify against the options

Checking the valid boundaries identified:

Option (B) 12.20.164.0/22: Correctly covers the ranges of Subnet 1 and Subnet 4.
Option (D) 12.20.168.0/22: Correctly covers the ranges of Subnet 3 and Subnet 2.

Final Answer:

The routing table can be most efficiently represented by the following aggregated prefixes: (B) 12.20.164.0/22 and (D) 12.20.168.0/22

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