GATE CSE 2014 Set 3 | Question: 26

Question

An IP router implementing Classless Inter-domain Routing (CIDR) receives a packet with address $131.23.151.76$. The router's routing table has the following entries:
$$\begin{array}{|c|c|c|} \hline \textbf {Prefix} & \textbf {Outer Interface Identifier} \\\hline \text {131.16.0.0/12} &  \text{3 } \\\hline \text{131.28.0.0/14} & \text{5} \\\hline \text{131.19.0.0/16} & \text{2} \\\hline \text{131.22.0.0/15} & \text{1} \\\hline \end{array}$$
The identifier of the output interface on which this packet will be forwarded is ______.

Comments

neel19 First one is taken.

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No, the ip with largest mask is selected.

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GO to the Root of Concept : Popular Misconceptions & what is Supernetting | Longest Match | Explained CLEARLY | With NOTES

Answer 1

The more number of 1st in Subnet Mask will be selected so keep this point in mind.

But before, look carefully at Second octet.

It is 00010111

In option A  it is 00010000

In option D it is 00010110

Clearly option D matches more number of bits than option A, also its Subnet mask has more number of 1's 

So option D is correct i.e. interface 1

 

Answer 2

131.19.0.0/ 16  Why is it not used?Can any one explain.

Comments

we need to consider "longest matching" here 131.16.0.0/ 12 and 131.22.0.0/ 15 both matched but /15 is longest matching it has more prefix so interface will be "1"