Design goal: 5 subnets, each needing at least 16 hosts. Which classful subnet mask should be used to satisfy these requirements?

Introduction / Context:
When choosing a subnet mask, you must meet both the required number of subnets and the minimum hosts per subnet. This is a classic capacity-planning exercise.

Given Data / Assumptions:

• Need at least 5 subnets.
• Each subnet needs at least 16 usable hosts.
• Looking at common Class C–style masks.

Concept / Approach:
Check host capacity first. For a mask to support at least 16 usable hosts, usable hosts = 2^(host_bits) - 2 must be >= 16. /27 (255.255.255.224) yields 30 hosts, /28 yields 14 hosts (insufficient). Then ensure number of subnets is enough.

Step-by-Step Solution:

Verification / Alternative check:
Even without subnet-zero (legacy), /27 still provides 6 usable subnets—still ≥5. With subnet-zero (modern default), you get 8.

Why Other Options Are Wrong:

• 255.255.255.240 (/28): Only 14 hosts.
• 255.255.255.248 (/29): Only 6 hosts.
• 255.255.255.192 (/26): Valid but overprovisions hosts and reduces subnet count flexibility unnecessarily for the stated minimums.
• 255.255.255.128 (/25): Similar overprovision; not needed.

Common Pitfalls:
Confusing “at least 16 hosts” with “16 addresses total.” Remember to subtract network and broadcast.

Final Answer:
255.255.255.224