GATE CSE 2012 | Question: 34, ISRO-DEC2017-32

Question

An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: $245.248.128.0/20$. The ISP wants to give half of this chunk of addresses to Organization $A$, and a quarter to Organization $B$, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to $A$ and $B$?

• A. $245.248.136.0/21 \text{ and } 245.248.128.0/22$
• B. $245.248.128.0/21 \text{ and }  245.248.128.0/22$
• C. $245.248.132.0/22 \text{ and }  245.248.132.0/21$
• D. $245.248.136.0/24 \text{ and }  245.248.132.0/21$

Comments

ip is starts from 245.248.128.0/20 not 254.248.128.0

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Awesome Question :-)

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This is solved by eliminating options and I understood it, is it also possible and safe to assume that if we use another approach (using VLSM on the available address) It depends upon question setters how they approached it?

Either they went on subnetting from

A – B – ISP or B – A – ISP or B – ISP – A

So we have to figure out which approach they used? (Although this is not efficient)

Answer 1

ans should be a) , the mask are /21 and/22 respectively, acc. to the half and quarter chunks requirement... and option b) can creat problem as address can get clashed between the two..

Answer 2

If take ip :245.248.128.0/21 then B have choice : 245.248.136.0/22 or 245.248.140.0/22

if A ip : 245.248.136.0/21 then B have choice : 245.248.128.0/22 or 245.248.132.0/22

 

So, correct answer A

Comments

very good explaination

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$\large {\color{Red} {Best\ Solution}}$

Answer 3

Answer is A

Duplicate of GATE 2012 -34

https://gateoverflow.in/1752/gate2012_34

Answer 4

Ans is A