How can one come up with reasonable IP subnet allocations? An ISP has following chunk of CIDR-based IP addresses available with it: 245.248.128.0/20
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?
Hints:


Try working out the binary representation of 245.248.136.0 (it's 32 bits) and of 245.248.128.0.
Re-read what the /21 and /22 notation means and how CIDR works.
What is the range of IP addresses included in 245.248.128.0/22?  in 245.248.136.0/21?


I think once you understand the concepts you'll find that this question is straightforward.



Advice: Rather than asking a question about this specific exercise, you might do better to ask a question about some concept you don't understand: e.g., what /N notation means, how CIDR works, etc.  But make sure to do your research first.  There's lots written on CIDR, /N notation, etc.  You can probably clear up your confusion on your own by doing proper reading on those concepts.