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 of 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

A.

I and II only

B.

II and III only

C.

III and IV only

D.

I and IV only

Solution:

Given: IP address is 202.61.0.0/17

and an organization requires a range of IP addresses to assign one to each of its 1500 computers.

For 1500 computers i.e. host, we need 11 bits host ID as 211≈ 2048. So, using 11 host ID bits, we can assign IP addresses to each of 1500 computers.

I. 202 . 61 . 01010100 . 00000000

II. 202 . 61 . 01101000 . 00000000

III. 202 . 61 . 01000000 . 00000000

IV. 202 . 61 . 10010000 . 00000000

In I, only the last 10 bits are 0's. Using 10 bits we cannot assign an IP address to 1500 computers. As 210 = 1024

In IV, the 17th bit from the start is 1 and since the given IP address is 202.61.0.0/17 .so, we cannot change the NID part of the original IP address.

So, options II and III are possible.