GATE CSE 2015 Set 2 | Question: 41

Question

Consider the following routing table at an IP router:
$$\begin{array}{|l|l|l|} \hline \textbf {Network No} & \textbf {Net Mask} & \textbf{Next Hop}  \\\hline \text {128.96.170.0} &  \text{255.255.254.0} & \text{Interface $0$} \\\hline\text {128.96.168.0} &  \text{255.255.254.0} & \text{Interface $1$} \\\hline\text {128.96.166.0} &  \text{255.255.254.0} & \text{R$2$}\\\hline \text {128.96.164.0} &  \text{255.255.252.0} & \text{R$3$}\\\hline \text {0.0.0.0} &  \text{Default} & \text{R$4$}\\\hline \end{array}$$
For each IP address in Group I Identify the correct choice of the next hop from Group II using the entries from the routing table above.
$$\begin{array}{l|ll} \hline \textbf {Group I} & \textbf {Group II}  \\\hline
\text {i) 128.96.171.92} &  \text{a) Interface 0} \\ \hline
\text {ii) 128.96.167.151} &  \text{b) Interface 1} \\\hline
\text {iii) 128.96.163.151} & \text{c) R$2$}\\ \hline
\text {iv) 128.96.164.121} & \text{d) R$3$}\\ \hline
\text {} & \text{e) R$4$}\\\hline \end{array}$$

• A. $\text{i-a, ii-c, iii-e, iv-d}$
• B. $\text{i-a, ii-d, iii-b, iv-e}$
• C. $\text{i-b, ii-c, iii-d, iv-e}$
• D. $\text{i-b, ii-c, iii-e, iv-d}$

Comments

iii matches with none. so it goes to default router.

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Similar questions:

https://gateoverflow.in/1051/gate2004-55

https://gateoverflow.in/3607/gate2006-it-63-isro2015-57

https://gateoverflow.in/2060/gate2014-3-26

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Yeah.. It doesn't matches with anyone

Answer 1

This simple concept of Bitwise ending Network Mask and IP address gives us network ID is being asked in GATE numerous times.

In question we are given with the network id and subnet mask. we are asked that which of the following IP addres belongs to which network?

 

As it is Classless

just look at third octet as the starting two octets are same in all options.

Approach:

Write the binary for 

163 - 10100011 matches with nothing so it will go through default 0.0.0.0

165 - 10100101   matches with    164 - 10100100              255.255.252.0

167 - 10100111   matches with    166 - 10100110              255.255.254.0

171 - 10101011   matches with    170 - 10101010              255.255.254.0

 

 

So option A is correct