A network engineer is provisioning subnets from the assigned address block 172.28.64.0/21 … — CompTIA Network+

A network engineer is provisioning subnets from the assigned address block $172.28.64.0/21$ using Variable Length Subnet Masking (VLSM). The subnets must be allocated sequentially starting from the lowest available IP address in the block to satisfy the following departmental requirements in descending order of size:

- Development: $480$ usable host IP addresses
- Sales: $120$ usable host IP addresses
- Support: $60$ usable host IP addresses
- Management: $14$ usable host IP addresses

After provisioning these four subnets with the smallest valid prefix length for each requirement, what is the decimal value of the fourth octet of the last usable host IP address in the Management department's subnet?

Answer: 206

Answer

The fourth octet of the last usable host IP address in the Management subnet is 206.

Sequentially allocating VLSM blocks from 172.28.64.0/21 yields: Development (172.28.64.0/23), Sales (172.28.66.0/25), Support (172.28.66.128/26), and Management (172.28.66.192/28). The broadcast address for Management is 172.28.66.207, making the last usable host IP address 172.28.66.206, which has a fourth octet of 206.

Step-by-Step Solution

Determine prefix length and address range for Development department (480 usable hosts)

Requires a /23 block (512 total IPs, 510 usable hosts). Range: 172.28.64.0/23 (172.28.64.0 to 172.28.65.255).

2^8 - 2 = 254 (insufficient), 2^9 - 2 = 510 (sufficient). 32 - 9 = /23 prefix length.

Determine prefix length and address range for Sales department (120 usable hosts)

Requires a /25 block (128 total IPs, 126 usable hosts). Starts at 172.28.66.0/25. Range: 172.28.66.0 to 172.28.66.127.

2^7 - 2 = 126 (sufficient for 120 hosts). 32 - 7 = /25 prefix length.

Determine prefix length and address range for Support department (60 usable hosts)

Requires a /26 block (64 total IPs, 62 usable hosts). Starts at 172.28.66.128/26. Range: 172.28.66.128 to 172.28.66.191.

2^6 - 2 = 62 (sufficient for 60 hosts). 32 - 6 = /26 prefix length.

Determine prefix length and address range for Management department (14 usable hosts)

Requires a /28 block (16 total IPs, 14 usable hosts). Starts at 172.28.66.192/28. Range: 172.28.66.192 to 172.28.66.207.

2^4 - 2 = 14 (exact requirement). 32 - 4 = /28 prefix length.

Extract the fourth octet of the last usable host address in Management's subnet

Broadcast address is 172.28.66.207, so the last usable host IP address is 172.28.66.206. Fourth octet is 206.

The last usable IP address in a subnet is always the broadcast address minus 1.

Key Concept

Variable Length Subnet Masking (VLSM) allocation and host calculation

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