GATE Computer Science (CS) 2020 Solved Papers

Question : 38 of 65

Marks: +1, -0

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 address space 202.61.0/17. 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

II and III only
I and II only
III and IV only
I and IV only

Validate

Solution:

No. of host

=1500

No. of host bits

=[\log _{2} 1500]=11

bits

∴

Total possible hosts

=2^{11}=2^{3} × 2^{8}

n

is the netmask bits,
Range of addresses is

=2^{32-n}

2^{11}=2^{32-π}

Available IP address

⇒ 202.61 .0 .0{/ }17

\table 0000 ,0000.0000,0000;0000,0111.1111,1111\}0.0-7.255

\table 0000 ,1000.0000,0000;0000,1111.1111,1111\}8.0\to15.255

So the IP address follow the pattern

0.0

7.255

8.0

15.255

16.0

23.255

⋮

64.0

to ...
⋮

104.0

. . .

∴ The possible IP addresses are 202.61.64.0/21 & 202.61.104.0/21

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