(b) Range of Addresses for Each Organisation and the Unallocated Range

Rule: allocate to each organisation the smallest block whose size is a power of 2 and is $\ge$ its requirement, so that the block stays CIDR-aligned (a block of $2^k$ addresses has prefix $/32-k$ and must start on a multiple of its own size).

Allocation proceeds from the start of the ISP block, each block being placed at an address aligned to its own size (e.g. a /23 always begins at an even value of the third octet, a /26 always at a multiple of 64 in the fourth octet).

Verification of the total

$$512 \times 2 + 256 \times 2 + 64 \times 3 = 1024 + 512 + 192 = 1728 \text{ addresses allocated}$$

Unallocated addresses

$$2048 - 1728 = 320 \text{ unallocated addresses}$$

Unallocated range = 80.70.62.192 to 80.70.63.255, which can itself be split into two aligned blocks:

• 80.70.62.192/26 — 64 addresses (80.70.62.192 – 80.70.62.255)
• 80.70.63.0/24 — 256 addresses (80.70.63.0 – 80.70.63.255)

Note: each allocated block also contains its own network address and broadcast address, so for example Org 1 gets 512 total addresses of which $512 - 2 = 510$ are usable host addresses — still more than the 500 it requires.