GATE CSE 2022 | Question: 45

Question

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

$$\begin{array} {|l|l|l|} \hline \text{Subnet Number} & \text{Subnet Mask} & \text{Next Hop} \\\hline 12.20.164.0 & 255.255.252.0 & \text{R1} \\\hline  12.20.170.0 & 255.255.254.0 & \text{R2} \\\hline 12.20.168.0 & 255.255.254.0 & \text{Interface 0} \\\hline 12.20.166.0 & 255.255.254.0 & \text{Interface 1} \\\hline \text{default} & & \text{R3} \\\hline \end{array}$$

Which of the following prefixes in $\text{CIDR}$ 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. $12.20.168.0/22$

Comments

Update: The GATE key didn’t change after being challenged 

This may be due to the series of RFCs published stating that the efficiency should be maintained while Aggregating Routes, to decrease packet movement in loops until expires (TTL = 0).  

Please find the RFC 1338
Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting: an Address Assignment and Aggregation Strategy", RFC 1338, DOI 10.17487/RFC1338, June 1992, <https://www.rfc-editor.org/info/rfc1338>

Section 2.1. Aggregation and its limitations
There are two situations that cause a loss of aggregation efficiency.

1. Organizations which are multi-homed
2. Organizations which move from one service provider to another.

discussed in detail, but they also mention 

aggregation efficiency gain can still be had for multi-homed sites (and, in general, for any site composed of multiple, logical IP network numbers) - by allocating a contiguous block of network numbers to the client (as opposed to multiple, independently represented network numbers) the client’s routing information may be aggregated into a single (net, mask) pair.

this RFC was published in 1992, hence, we can consider allocating continuous blocks while Aggregating, Though we can combine, if we keep on combining it will eventually become 0.0.0.0/0 

“BUT” I don't get why would anyone aggregate different interfaces 

so I feel this question is “Marks to ALL” worthy (and I would suggest anyone not to spend too much time on this one)

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video soln 

https://youtu.be/bCiShDRd3tk?si=Qhkj3SEdJ_osowwI

is this video solution is correct ?

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Why option A is wrong ?

Answer 1

Write the changing bits in binary and select the overall range from the options accordingly.