Q6(b) · VLSM groups

Q6(b). An ISP is allocated a block of addresses starting with 188.15.0.0/16 (65,536 addresses). The ISP needs to distribute these addresses among three customer groups as follows: i) The first group consists of 64 customers; each requires 256 addresses. ii) The second group consists of 128 customers; each requires 128 addresses. iii) The third group consists of 64 customers; each requires 128 addresses. Design the sub blocks and determine the first address and last address of first customer and last customer for each group. Calculate how many addresses are still available after these allocations. [10 marks]

Answer

Given

188.15.0.0/16; Group 1: 64 customers × 256 addresses; Group 2: 128 customers × 128 addresses; Group 3: 64 customers × 128 addresses

To find

Each customer prefix, every group range, first/last customer blocks, and unallocated addresses.

Formula and why

LaTeX: p=32-\log_2(\text{addresses per customer}),\quad A_{group}=n\times A_{customer}

These are requested total addresses, not usable-host counts. Allocate customer blocks on their prefix boundary. A group with a non-power-of-two customer count need not be forced into one larger, wasteful CIDR aggregate.

Step 1. Compute sizes and allocate consecutive aligned customer blocks

Group	Customers	Addresses each	Customer prefix	Used	Group range
1	64	256	/24	16384	188.15.0.0–188.15.63.255
2	128	128	/25	16384	188.15.64.0–188.15.127.255
3	64	128	/25	8192	188.15.128.0–188.15.159.255

Step 2. State first and last customer ranges

Group	First customer	Last customer
1	188.15.0.0–188.15.0.255	188.15.63.0–188.15.63.255
2	188.15.64.0–188.15.64.127	188.15.127.128–188.15.127.255
3	188.15.128.0–188.15.128.127	188.15.159.128–188.15.159.255

Step 3. Count the unused addresses

Allocated = 16384 + 16384 + 8192 = 40960. Unused = 65536 − 40960 = 24576.

Unused range: 188.15.160.0–188.15.255.255.