If you want to create 8 usable subnets from a Class C network using the subnet mask 255.255.255.224, which global configuration command must be enabled on a Cisco router to allow the use of all subnets, including subnet zero?

Introduction / Context:
This question examines your knowledge of older Cisco IOS behavior regarding subnet zero and the configuration required to use all possible subnets in a classful network. Historically, some textbooks and default configurations discouraged the use of the first and last subnet, but modern best practice allows using all subnets to conserve address space, which requires a specific command on some platforms.

Given Data / Assumptions:
- The base network is a Class C network.
- The subnet mask 255.255.255.224 is used, which is a /27 prefix and creates 8 possible subnets in a Class C range.
- We want to use all 8 subnets, including subnet zero.
- The question is about a Cisco router global configuration command that enables this behavior.

Concept / Approach:
With a Class C network and a /27 mask, the number of subnets is 2^(number of borrowed bits). Moving from /24 to /27 borrows 3 bits, so there are 2^3 = 8 possible subnets. Older documentation sometimes recommended not using subnet zero (all subnet bits 0) to avoid confusion, which would leave only 7 usable subnets. On Cisco routers, the global configuration command ip subnet-zero enables the use of subnet zero, allowing all 8 subnets to be used. Modern IOS versions usually have this enabled by default, but it remains an important exam concept.

Step-by-Step Solution:
Step 1: Recognize that 255.255.255.224 corresponds to /27 and creates 8 subnets in a Class C network.Step 2: Recall that without subnet zero, only 7 of the 8 subnets would be considered usable.Step 3: Identify that Cisco introduced the ip subnet-zero command so that administrators can use the first subnet (subnet zero) just like other subnets.Step 4: Note that ip classless is related to classless routing behavior, not specifically to allowing subnet zero.Step 5: Conclude that Router(config)#ip subnet-zero is the required command to allow all 8 subnets.

Verification / Alternative check:
You can verify by reviewing sample configurations in Cisco training material. For example, if you have a 192.168.1.0/24 network and you apply a /27 mask, the subnets start at 192.168.1.0, 192.168.1.32, 192.168.1.64, and so on. Without ip subnet-zero, some older systems would consider 192.168.1.0/27 off limits, but with ip subnet-zero enabled, it becomes a normal usable subnet, increasing efficiency.

Why Other Options Are Wrong:
The ip classless command affects how the router handles routes that do not exactly match classful boundaries but does not control use of subnet zero. The ip version 6 command is not a valid Cisco global command in this form and relates to IPv6, not IPv4 subnetting. The ip unnumbered command is used to borrow an IP address from another interface, not to control subnet usage. Disabling IP routing would break the router function entirely and is unrelated to subnet allocation.

Common Pitfalls:
Students sometimes confuse ip classless with ip subnet-zero because both refer to classful behavior. Another pitfall is to assume that subnet zero is always forbidden, which is outdated. Modern designs routinely use subnet zero, and many IOS releases enable ip subnet-zero by default, but exams can still ask about the command explicitly, so it is important to recognize it.

Final Answer:
The required command is Router(config)#ip subnet-zero, which allows all 8 subnets, including subnet zero, to be used.