Latest Gate Aptitude Question SOLUTION: 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

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(#M40029657) GATE question GATE-CS Keep an EYE

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?

option
(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

Asked In Gate Rats (10 years ago)

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Venkat Reddy  (10 years ago)

Since routing prefix is 20, the ISP has 2^(32-20) or 2^12 addresses. Out of these 2^12 addresses, half (or 2^11) addresses have to be given to organization A and quarter (2^10) addresses have to be given to organization B. So routing prefix for organization A will be 21. For B, it will be 22. If we see all options given in question, only options (A) and (B) are left as only these options have same number of routing prefixes. Now we need to choose from option (A) and (B).

half of 4096 host addresses must be given to organization A, we can set 12th bit to 1 and include that bit into network part of organization A, so the valid allocation of addresses to A is 245.248.136.0/21.

Now for organization B, 12th bit is set to '0' but since we need only half of 2048 addresses, 13th bit can be set to '0' and include that bit into network part of organization B so the vaild allocation of addresses to B is 245.248.128.0/22

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