The GATE Grind

GATE 2020 CS – Question 48

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

An organization requires a range of IP addresses to assign one to each of its 1500 computers. The organization has approached an Internet Service Provider (ISP) for this task. The ISP uses CIDR and serves the requests from the available IP address space 202.61.0.0/17. The ISP wants to assign an address space to the organization which will minimize the number of routing entries in the ISP's router using route aggregation. Which of the following address spaces are potential candidates from which the ISP can allot any one to the organization?

I. 202.61.84.0/21

II. 202.61.104.0/21

III. 202.61.64.0/21

IV. 202.61.144.0/21

  1. I and II only
  2. II and III only
  3. III and IV only
  4. I and IV only

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Correct answer: (B) II and III only

Explanation

1500 hosts need 2048 addresses, i.e. a /21 block (third octet a multiple of 8). The ISP's space 202.61.0.0/17 spans third octets 0-127. 84 is not a multiple of 8 (invalid) and 144 lies outside the /17. Valid candidates: 104.0/21 and 64.0/21.