An ISP has following chunk of CIDR-based IP addresses available with it:
The ISP wants to give half of this chunk 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 and 245.248.128.0/22
B) 245.248.128.0/21 and 245.248.128.0/22
C) 245.248.132.0/21 and 245.248.132.0/21
D) 245.248.136.0/24 and 245.248.132.0/21
It is easy to make out that option 'C' and 'D' are wrong. Option 'B' is not possible due to address overlapping. The answer is 'A' for sure, but I'm unable to understand how. How is 245.248.136.0/21 and 245.248.128.0/22 calculated? I've been trying to solve this the whole day but found no solution to it, any help on how to solve this problem would be appreciated.
Also, how can I determine the range of addresses that can be allocated to organizations A and B?