Question

An Internet Service Provider (ISPP) 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 of Organization B, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to A and B?

• A. 245.248.128.0/21 and 245. 248.128.0/22
• B. 245.248.132.0/22 and 245.248.132.0/21
• C. 245.248.136.0/24 and 245.248.132.0/21
• D. 245.248.136.0/21 and 245.248.128.0/22

Answer 1

The correct option is D 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.
$\therefore$ Two choices for A
$\Rightarrow$ 245.248.1000 0000.0/21
245.248.1000 1000.0/21
$\therefore$ A can be assigned
$\Rightarrow$ 245.248.136.0/21
245.248.128.0/22
Now, one fourth needs to be assigned to B, hence its mask should be of 22 bits. But if we look at choice (b), the host in A and B are conflicting, therefore option (a) is correct.