GATE | GATE CS 2012 | Question 65
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?
245.248.136.0/21 and 245.248.128.0/22
245.248.128.0/21 and 245.248.128.0/22
245.248.132.0/22 and 245.248.132.0/21
245.248.136.0/24 and 245.248.132.0/21
Explanation : The correct option is A – 245.248.136.0/21 and 245.248.128.0/22
It has 20 bit mask, so total number of hosts are 2^12 hosts.
It wants half of the chunk to A, therefore mask of A should be 21 bits, so it will contain 2^11 hosts.
∴ Two choices for A
⇒ 245.248.1000 0000.0/21
∴ A can be assigned
Now, one fourth needs to be assigned to B, hence its mask should be of 22 bits. But if we look at other choice, the host in A and B are conflicting, therefore option (A) is correct.
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