An Internet Service Provider (ISP) has the following chunk of CIDR-based IP…

201220252017

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?

  1. A.

    245.248.136.0/21 and 245.248.128.0/22

  2. B.

    245.248.128.0/21 and 245.248.128.0/22

  3. C.

    245.248.132.0/22 and 245.248.132.0/21

  4. D.

    245.248.136.0/24 and 245.248.132.0/21

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Correct answer: A

Key insight: split the /20 into subnets by size. A /20 has 4096 addresses; half is a /21 (2048 addresses) and a quarter is a /22 (1024 addresses).

  • Step 1: Identify /21 boundaries inside 245.248.128.0/20. The two /21s are 245.248.128.0/21 (covers 245.248.128.0–245.248.135.255) and 245.248.136.0/21 (covers 245.248.136.0–245.248.143.255).

  • Step 2: Subdivide one /21 into /22s if needed. 245.248.128.0/21 contains two /22s: 245.248.128.0/22 (245.248.128.0–245.248.131.255) and 245.248.132.0/22 (245.248.132.0–245.248.135.255).

  • Step 3: Allocate accordingly. Give 245.248.136.0/21 (a half) to Organization A and 245.248.128.0/22 (a quarter) to Organization B. The remaining block 245.248.132.0/22 stays with the ISP.

Therefore the valid allocation is 245.248.136.0/21 and 245.248.128.0/22 because they are correctly sized, aligned, non-overlapping, and lie within 245.248.128.0/20.

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