GATE IT 2005 | Question: 76

Question

A company has a class $C$ network address of $204.204.204.0$. It wishes to have three subnets, one with $100$ hosts and two with $50$ hosts each. Which one of the following options represents a feasible set of subnet address/subnet mask pairs?

• A. $204.204.204.128/255.255.255.192$
  $204.204.204.0/255.255.255.128$
  $204.204.204.64/255.255.255.128$
• B. $204.204.204.0/255.255.255.192$
  $204.204.204.192/255.255.255.128$
  $204.204.204.64/255.255.255.128$
• C. $204.204.204.128/255.255.255.128$
  $204.204.204.192/255.255.255.192$
  $204.204.204.224/255.255.255.192$
• D. $204.204.204.128/255.255.255.128$
  $204.204.204.64/255.255.255.192$
  $204.204.204.0/255.255.255.192$

Comments

DBA: keep network id and subnet id bits same, make all host bits as 1s

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I am having a doubt here.

Is it a valid question as broadcast address for network will be 204.204.204.255 and for subnet a with 100 hosts will also be same similiar to this network address is also same for one of 50 hosts subnet which is having 00 as subnetting bits

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But how can network id and subnet id be same

Answer 1

MSB in last 8 bits is in use to get subnet since it is class c ip

10000000 / 128(mask)subnet id bit(10) --->subnet1 ( since need 100 host so we assign max 128 here so 28 gone waste).
01000000/192( mask) subnet id bit(01)--->subnet2 ( since need 50 host so we assign max 64 here so 14 gone waste). here we can go for option c b/c we get coolision of ip address since subnet 1 also have 11000000 and subnet 2 also . so here D is answer
00000000 /192( mask)subnet id bit(00)--->subnet2(0)  ( since need 50 host so we assign max 64 here so 14 gone waste).

D is answer.

Comments

In your first case, 1000000 or 128 the subnet ID would be just 1 and not 10. If it would have been 10 then we would have not got 128 addresses or hosts. It would have been just 64 hosts.

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yupp..me too have the same issue..instead of 10 the subnet id would be just 1 ...!!

Answer 2

I have drawn two possible cases here

Case 2 is our answer which is option D

 

Comments

But how can network id and subnet id be same

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with practice

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@StoneHeart then ans can be A also rt?