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Module 3: Network Layer & Routing

20202m

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?

(i) 245.248.136.0/21 and 245.248.128.0/22
(ii) 245.248.128.0/21 and 245.248.128.0/22
(iii) 245.248.132.0/22 and 245.248.132.0/21
(iv) 245.248.136.0/22 and 245.248.132.0/21

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Answer: (i) 245.248.136.0/21 and 245.248.128.0/22

Why: The block 245.248.128.0/20 spans 245.248.128.0 – 245.248.143.255 (4096 addresses), so a half is a /21 (2048 addrs) and a quarter is a /22 (1024 addrs). Option (i) gives A the upper half 136.0 – 143.255 and B the first quarter 128.0 – 131.255: both lie inside the /20 and do not overlap. In (ii) both start at 128.0, so B's /22 is nested inside A's /21 (overlap); in (iii) the sizes are swapped (A gets only 1024) and they overlap; in (iv) A gets a /22 (a quarter, not a half) and overlaps B's /21.

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