GATE 2012 CS (CS1) – Question 44

Computer Networks · IPv4: Addressing, CIDR, Fragmentation and NAT · 2 marks · Multiple choice

An ISP holds CIDR block 245.248.128.0/20. Half goes to A, one quarter to B, and the rest is retained. Which A,B allocation is valid?

245.248.136.0/21; 245.248.128.0/22
245.248.128.0/21; 245.248.128.0/22
245.248.132.0/22; 245.248.132.0/21
245.248.136.0/24; 245.248.132.0/21

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Show answer and explanation

Correct answer: (A) 245.248.136.0/21; 245.248.128.0/22

Explanation

Treat the printed addresses as the question’s abstract CIDR block. A /20 spans third octets 128–143. Half requires /21 and one quarter /22. Option A gives A=136–143 and B=128–131, aligned and disjoint with 132–135 retained. Other options overlap, misalign or use wrong sizes. Thus A; 245.x is reserved address space, not a public-allocation example.

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