Gate2020 cs Q63

63An 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 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

Option : A
Explanation :
No. of host = 1500
No. of host bits = [log₂ 1500] = 11 bits
∴ Total possible hosts = 2¹¹ = 2³ × 2⁸
n is the netmask bits,
Range of addresses is = 2^{32 – n}
2¹¹ = 2^32-n
Available IP address ⇒ 202.61.0.0/17

So the IP address follow the pattern
0.0 to 7.255
8.0 to 15.255
16.0 to 23.255
:
64.0 to ...
:
104.0 to ...
∴ The possible IP addresses are 202.61.64.0/21 & 202.61.104.0/21

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