An Internet Service Provider (𝐼𝑆𝑃) has following chunk of 𝐶𝐼𝐷𝑅−based…
2018
An Internet Service Provider (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.
245.248.132.0/22 and 245.248.132.0/21
- D.
245.248.136.0/24 and 245.248.132.0/21
Attempted by 104 students.
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Correct answer: A
Final allocation: 245.248.136.0/21 to organization A; 245.248.128.0/22 to organization B; ISP retains 245.248.132.0/22.
Compute sizes: a /20 contains 2^(32-20)=4096 addresses. Half is 2048 addresses (a /21). A quarter is 1024 addresses (a /22).
List aligned subnets inside 245.248.128.0/20:
Valid /21 subnets: 245.248.128.0/21 (covers .128.0–.135.255) and 245.248.136.0/21 (covers .136.0–.143.255).
Valid /22 subnets: 245.248.128.0/22, 245.248.132.0/22, 245.248.136.0/22, 245.248.140.0/22.
Choose non-overlapping allocations that match the requested sizes: assign 245.248.136.0/21 (2048 addresses) as the half; assign 245.248.128.0/22 (1024 addresses) as the quarter. The remaining 245.248.132.0/22 (1024 addresses) stays with the ISP.
Verify: the chosen /21 and /22 do not overlap and all lie within 245.248.128.0/20, satisfying the requirement.
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