VLSM practice — /28 subnets in 192.168.10.0/28 with zero subnet disallowed If you require the <em>eighth</em> usable /28 subnet (zero subnet not considered valid) in 192.168.10.0/28 and must assign the last usable IP to interface E0, which exact host address should you configure?

Introduction / Context:
Calculating host ranges inside small subnets is a staple skill when allocating addresses on point-to-point and access networks. Here, we must skip the zero subnet and then choose the eighth /28 and its last usable IP for a router interface.

Given Data / Assumptions:

• Base network: 192.168.10.0.
• Mask: /28 (255.255.255.240), block size 16 addresses per subnet.
• Zero subnet not valid for this question.
• We want the last usable host of the eighth /28.

Concept / Approach:
With /28, each subnet spans 16 addresses: .0–.15, .16–.31, .32–.47, ... The zero subnet (.0/28) is excluded, so counting starts at .16/28 as the first usable. The last usable in any /28 is subnet_end − 1 (because the broadcast is subnet_end).

Step-by-Step Solution:
Enumerate /28 subnets (excluding .0): .16 (1), .32 (2), .48 (3), .64 (4), .80 (5), .96 (6), .112 (7), .128 (8).Identify the eighth subnet: 192.168.10.128/28 covering .128–.143.Usable hosts are .129–.142; broadcast is .143.Select the last usable: 192.168.10.142.

Verification / Alternative check:
Compute broadcast as next subnet start minus one: next subnet after .128 is .144, so broadcast is .143; hence last usable is .142, confirming our choice.

Why Other Options Are Wrong:

• B/E: Belong to different /28 ranges (.64–.79 and .112–.127 respectively), not the eighth subnet’s last host.
• C: Different third octet entirely (192.168.100.x), invalid network.
• D: .143 is the broadcast of the eighth /28, not a usable host.

Common Pitfalls:
Forgetting to exclude the zero subnet when instructed or mistakenly using the broadcast address as the last host.

Final Answer:
192.168.10.142