CIDR Allocation from 80.70.56.0/21

(i) Number and range of addresses in the ISP block

• Prefix length = 21 → number of addresses = $2^{32-21} = 2^{11} =$ 2048.
• Block starts at 80.70.56.0. Third octet = 56 = 00111000; a /21 keeps the first 5 bits of the third octet fixed, so the third octet runs from 56 (56 & 248) to 63 (56 | 7).
• Range: 80.70.56.0 – 80.70.63.255 (usable hosts: 2048 − 2 = 2046).

(ii) Allocation — smallest power-of-two block ≥ the requirement

Workings: 500 → next power of 2 = 512 = $2^9$ → 9 host bits → /23; 250 → 256 = $2^8$ → /24; 50 → 64 = $2^6$ → /26. Blocks are assigned top-down, contiguously, without overlap, each aligned on its own size boundary (56/58/60/61/62 are all multiples of 2, 4 and 8 respectively as needed).

Allocated total = 512 + 512 + 256 + 256 + 64 + 64 + 64 = 1728 addresses.

Unallocated range: 80.70.62.192 – 80.70.63.255 = 320 addresses
= 80.70.62.192/26 (64) + 80.70.63.0/24 (256) — available for future customers or the ISP's own infrastructure (router interfaces, gateways).

Check: 1728 + 320 = 2048 = the block size ✔