Consider the routing table with entries for subnets 12.20.164.0/255.255.252.0(R1), 12.20.170.0/255.2

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GATE-CS is a competitive examination conducted in India. This question is from the 2022 GATE-CS Computer Networks paper.

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Question

Consider the routing table with entries for subnets 12.20.164.0/255.255.252.0(R1), 12.20.170.0/255.255.254.0(R2), 12.20.168.0/255.255.254.0(Interface 0), 12.20.166.0/255.255.254.0(Interface 1), default(R3). Which CIDR prefixes can collectively correctly aggregate all subnets?

Accepted Answer

Correct Answer: (B) and (D)Concept -- Route Aggregation / Supernetting (from Kurose and Ross, Computer Networking):The subnets are:12.20.164.0/22 (255.255.252.0): covers 12.20.164.0 to 12.20.167.25512.20.170.0/23 (255.255.254.0): covers 12.20.170.0 to 12.20.171.25512.20.168.0/23 (255.255.254.0): covers 12.20.168.0 to 12.20.169.25512.20.166.0/23 (255.255.254.0): covers 12.20.166.0 to 12.20.167.255Groups: 12.20.164-167 range can be aggregated as 12.20.164.0/22 (covers .164.0 to .167.255 = /22 with mask 255.255.252.0). 12.20.168-171 range can be aggregated as 12.20.168.0/22 (covers .168.0 to .171.255). Together B (12.20.164.0/22) and D (12.20.168.0/22) cover all four subnets exactly. Option (C) 12.20.164.0/21 would cover .164.0 to .171.255 -- all 8 subnets -- but would over-aggregate (include addresses not in any specific subnet in the table). Option (A) /20 covers an even larger block. The correct minimal aggregation is (B) and (D).

Consider the routing table with entries for subnets 12.20.164.0/255.255.252.0(R1), 12.20.170.0/255.255.254.0(R2), 12.20.168.0/255.255.254.0(Interface 0), 12.20.166.0/255.255.254.0(Interface 1), default(R3). Which CIDR prefixes can collectively correctly aggregate all subnets?

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Consider the routing table with entries for subnets 12.20.164.0/255.255.252.0(R1), 12.20.170.0/255.255.254.0(R2), 12.20.168.0/255.255.254.0(Interface 0), 12.20.166.0/255.255.254.0(Interface 1), default(R3). Which CIDR prefixes can collectively correctly aggregate all subnets?

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