GATE Overflow Test Series | Computer Networks | Test 1 | Question: 28

Question

The subnet mask for a particular network is $255.255.0.0.$ Which of the following pairs of IP addresses could belong to this network? (Mark all the appropriate choices)

• A. $158.59.195.66$ and $158.59.131.33$
• B. $16.39.22.2$ and $16.39.48.9$
• C. $191.28.106.43$ and $191.29.202.54$
• D. $35.8.129.55$ and $35.9.129.59$

Comments

I thinck C will also corrct as Ip addresses in C option belong to Class B so its subnet mask is also be same as given in question

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@Deepak Poonia sir here in this question option B should also be the answer isn’t it? Because they bith also have same network address as per class B subnet mask given in the question? @GO Classes

Answer 1

The given subnet is of class B as it has $16$ Network ID bits. So, option B is ruled out.
    
We get the network number by the bitwise AND of IP and subnet mask. Options C and D are not answers as the second byte of IP address differs whereas second byte of subnet is all $1s.$

 Let's see option A.

 $158.59.195.66 \;\&\; 255.255.0.0 = 158.59.0.0$
 $158.59.131.33 \;\&\; 255.255.0.0 = 158.59.0.0$
   
 Thus we get the same network number.

Comments

A,B should also be true because they give the same Network ids.

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Why is option B is incorrect and A is correct?

Can we consider this case? :-

There is also possibility that subnet mask belongs to class A such that in subnet mask, 1’s in second octat are for borrowed bits of subnet id.

If this case is considered, then option A becomes incorrect as it has ‘class B’ IP addresses.    

Please clear this doubt

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@gatecse sir, please clarify why we are by default assuming classfull-adressing here. Is there any standard result about this?