The answer is 255.255.255.224, which corresponds to a /27 subnet mask, because it provides exactly 30 usable IP addresses—the smallest subnet that accommodates 30 devices with minimal waste. This works because a /27 network contains 32 total addresses, but two are reserved for the network ID and broadcast address, leaving 30 for hosts. On the CompTIA A+ Core 1 220-1201 exam, this question tests your ability to match subnet sizes to device counts, often appearing in DHCP configuration scenarios where conserving IPs is critical. A common trap is choosing a /28 (14 usable IPs) which is too small, or a /26 (62 usable IPs) which wastes addresses. To remember, think: 30 devices need 32 addresses, and 32 is 2^5, so the host bits are 5, meaning the network bits are 27 (32 minus 5). A quick mnemonic: “30 devices, /27 is the heaven—no waste, just right.”
220-1101 IP Addressing Practice Question
This 220-1201 practice question tests your understanding of ip addressing. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A technician is configuring a small branch office network with 30 devices. The network uses a single router with DHCP enabled. To conserve IP addresses, the technician decides to use a subnet that provides exactly enough usable addresses for the devices, with minimal waste. Which subnet mask should the technician choose?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
255.255.255.224
A /27 subnet (255.255.255.224) provides 30 usable addresses (32 total minus 2 reserved). This is the smallest subnet that accommodates 30 devices without wasting many addresses.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
255.255.255.0
Why it's wrong here
Incorrect. A /24 provides 254 usable addresses, which is far more than needed and wastes addresses.
✓
255.255.255.224
Why this is correct
Correct. A /27 provides 30 usable addresses, exactly enough for 30 devices with minimal waste.
Incorrect. A /28 provides only 14 usable addresses, which is insufficient for 30 devices.
✗
255.255.255.192
Why it's wrong here
Incorrect. A /26 provides 62 usable addresses, which is more than needed but not the most efficient choice.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 220-1201 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.
IP Addressing — This question tests IP Addressing — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: 255.255.255.224 — A /27 subnet (255.255.255.224) provides 30 usable addresses (32 total minus 2 reserved). This is the smallest subnet that accommodates 30 devices without wasting many addresses.
What should I do if I get this 220-1201 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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