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 addresses for 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

Correct answer: 245.248.136.0/21 and 245.248.128.0/22

Solution

The block 245.248.128.0/20 spans 4096 addresses. Half of it is a /21 block and a quarter is a /22 block; both must begin at valid subnet boundaries and be non-overlapping. 245.248.128.0/22 is valid for B, and 245.248.136.0/21 is a valid non-overlapping half-block for A.

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