Choosing the last usable IP in the first valid /28 subnet Given network 192.168.10.0/28, the zero subnet is not considered valid. Using the first valid /28 subnet, what should be the IP address of interface S0 if you must use the last available host address in that range?

Introduction / Context:
Subnetting questions often test your ability to enumerate ranges and pick boundary addresses accurately. Here you are given a Class C–like network with a /28 mask and a constraint that the zero subnet (the first numerically) is not valid. You must identify the first valid subnet's range and then choose the last usable host address within that range for the S0 interface.

Given Data / Assumptions:

• Base network: 192.168.10.0/28.
• Zero subnet is not considered valid for this question's context.
• We need the last usable host address in the first valid /28 subnet.

Concept / Approach:

/28 corresponds to a mask of 255.255.255.240. The block size is 16 in the last octet (since 256 - 240 = 16). Subnets increment by 16: .0, .16, .32, .48, … Each /28 provides 16 addresses: 1 network, 14 usable hosts, 1 broadcast. If the zero subnet .0/28 is disallowed, the first valid subnet is .16/28 whose usable range is .17 through .30, with .31 as broadcast.

Step-by-Step Solution:

Verification / Alternative check:

Confirm broadcast of the .16/28 subnet is .31 by formula: last address in the block (16 + 15 = 31). Therefore the last usable host is one less than broadcast, .30. A quick ping sweep or lab test will validate reachability assignments within the .17–.30 span.

Why Other Options Are Wrong:

• .24 sits inside the first valid /28, but it is not the last usable address.
• .62 and .127 are in different, larger subnets (/26 and /25 boundaries respectively), not in the first /28 block after .0.
• .14 is in the zero subnet, which the problem statement excludes.

Common Pitfalls:

• Mistaking the block size or mixing broadcast and last-usable addresses.
• Forgetting the constraint that the zero subnet is considered invalid here.

Final Answer:

192.168.10.30