Cisco IOS requirement: To allow the use of all 8 subnets with a Class C mask of 255.255.255.224 (/27), which configuration command must be enabled?

Introduction / Context:
Older IOS releases required an explicit command to allow using the first and last subnets (so-called “subnet-zero”). Modern IOS enables it by default, but many exams still test the concept.

Given Data / Assumptions:

• Mask: 255.255.255.224 (/27) on a Class C network.
• Goal: Use all 8 possible /27 subnets (including subnet-zero and the all-ones subnet).

Concept / Approach:
Total subnets with 3 borrowed bits is 2^3 = 8. Historically, some implementations avoided using the first and last subnets. The IOS command ip subnet-zero permits their use, enabling the full count.

Step-by-Step Solution:

Verification / Alternative check:
On modern IOS, verify with show running-config or confirm addressing acceptance when configuring interfaces.

Why Other Options Are Wrong:

• ip classless: Controls classless routing behavior, not subnet-zero usage.
• no ip classful: Not a valid IOS command.
• ip unnumbered: Interface addressing feature, unrelated.
• ip route-cache: CEF/fast switching, unrelated.

Common Pitfalls:
Confusing classless routing with subnet-zero; assuming all IOS versions always use subnet-zero without checking.

Final Answer:
Router(config)# ip subnet-zero