The IPv4 network 192.168.100.0/27 is configured on a router for subnetting. Assuming that the ip subnet zero feature is enabled, which option correctly states how many usable subnets are available and how many usable host addresses exist in each subnet?

Introduction / Context:
This subnetting question checks your understanding of how prefix length, number of subnets, and usable host addresses per subnet are related when ip subnet zero is enabled on a router. In many entry level networking exams, this specific type of problem appears to verify that you can translate a slash notation mask such as 27 into the number of available subnets and hosts that you can safely assign to end devices.

Given Data / Assumptions:
- Network address is 192.168.100.0/27 which is a Class C private IPv4 network.
- The router is using classless routing with ip subnet zero enabled, so the first and last subnets are allowed.
- We are interested in the number of usable subnets and the number of usable host addresses per subnet, not total addresses including network and broadcast.

Concept / Approach:
With a Class C style address, a /24 prefix would normally represent a single subnet with 256 total addresses. Moving the prefix from /24 to /27 borrows 3 bits from the host portion and adds them to the network portion. The number of subnets is 2 raised to the power of borrowed bits. The number of total addresses per subnet is 2 raised to the power of the remaining host bits. Usable hosts are total addresses minus the network and broadcast addresses in each subnet.

Step-by-Step Solution:
Step 1: For a Class C style network the default mask is /24, so the /27 prefix borrows 3 bits from the host part.Step 2: Calculate the number of subnets as 2 ^ 3 which equals 8 possible subnets when ip subnet zero is allowed.Step 3: The host portion now has 5 bits remaining since 32 total bits minus 27 network bits equals 5 host bits.Step 4: Total addresses in each subnet equal 2 ^ 5 which is 32 addresses per subnet.Step 5: Subtract 2 addresses in every subnet for the network and broadcast addresses so usable hosts per subnet equal 32 minus 2 which is 30.

Verification / Alternative check:
A quick alternative check is to remember that a /27 block size in the last octet is 32. Listing subnets as 192.168.100.0, 32, 64, 96, 128, 160, 192, and 224 clearly shows 8 different network IDs. Within any one of these ranges, for example 192.168.100.32 to 192.168.100.63, the first address is the network ID and the last is the broadcast, leaving 30 usable host addresses in between.

Why Other Options Are Wrong:
7 usable subnets each with 32 host addresses is incorrect because there are 8 subnets when ip subnet zero is enabled and you cannot use all 32 addresses as hosts. 7 usable subnets each with 24 usable host addresses is incorrect because the math for a /27 does not produce 24 hosts. 8 usable subnets each with 32 host addresses ignores the network and broadcast addresses and therefore overstates the usable host count. 4 usable subnets each with 64 host addresses relates more to a /26 design than to a /27 network.

Common Pitfalls:
Students often forget to subtract 2 addresses per subnet for the network and broadcast addresses and mistakenly quote total addresses per subnet instead of usable hosts. Another common error is to still exclude subnet zero even when ip subnet zero is explicitly allowed, which reduces the subnet count incorrectly to 7. It is also easy to mix up designs for /26 or /28 with a /27 mask because those all occur frequently in practice tests.

Final Answer:
The correct statement is that there are 8 usable subnets each with 30 usable host addresses per subnet.