Project ForgeAn 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
- AI and II only
- II and III only
- CIII and IV only
- DI and IV only
Explanation
1500 addresses need a block of at least 2048 addresses, i.e. a /21 (2¹¹=2048). A valid /21 subnet must start at a third-octet value that's a multiple of 8 (2048/256=8), AND must lie fully within the given /17 space, which covers third-octet values 0-127 only (since /17 = 16 bits + 1 bit, giving a 128-value range in the third octet: 202.61.0.0–202.61.127.255). Check each: I) 84/8=10.5 — not a multiple of 8, invalid alignment. II) 104/8=13 — valid alignment, and 104<128, within range. III) 64/8=8 — valid alignment, within range. IV) 144 > 127 — outside the /17 space entirely, invalid. So II and III only.
Source: GATE CS 2020 (2020)
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