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

---

Awesome Question :-)

---

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

$245.248.128.0/20$

 

Underlined is NID; rest is HID

$\underline{245.248.1000}0000.00000000$

---

To give half the chunk to someone, we'd need $1$ Subnet bit.

 

First portion is NID, Mid portion (single bit) is SID, last portion is HID.

$\underline{245.248.1000}\ \ \underline0 \ \  \underline{000.00000000}$

 

If I don't touch the SID bit; I get $245.248.128.0/21$ as the address.

If I change the bit to 1, I get: $245.248.136.0/21$ as the address.

 

Since $245.248.132.0/21$ isn't even possible, Options C and D are out of consideration.

And Option B is just the same address, it isn't really possible to give them to two different organisations.

So, Option A is the answer.

 

But, let's see further.

---

Now, for giving away a quarter of the chunk, I need $2$ SID bits.

So,

$\underline{245.248.1000}\ \ \underline{00} \ \  \underline{00.00000000}$

 

If I keep my SID as $10$, I get

$245.248.136.0/22$

 

Hence, Option A.

Comments

If I keep my SID as 10, I get

 

245.248.136.0/22

Here SID will be 00, because 10  and 11 is taken by subnet A. and you will get 245.248.128.0/22