- A
255.255.255.0
Why wrong: Incorrect. A /24 provides 254 usable addresses, which is far more than needed and wastes addresses.
- B
255.255.255.224
Correct. A /27 provides 30 usable addresses, exactly enough for 30 devices with minimal waste.
- C
255.255.255.240
Why wrong: Incorrect. A /28 provides only 14 usable addresses, which is insufficient for 30 devices.
- D
255.255.255.192
Why wrong: Incorrect. A /26 provides 62 usable addresses, which is more than needed but not the most efficient choice.
Question 282 of 1,020
Subnet Mask for 30 Devices: /27 (255.255.255.224) Provides Exactly 30 Usable IPs
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?
Quick Answer
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.”
Answer choices
Why each option matters
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
The network requires exactly 30 usable IP addresses. A /27 subnet (mask 255.255.255.224) provides 32 total addresses, with 30 usable (2^5 - 2 = 30), matching the requirement with minimal waste. This is the most efficient choice because it supplies the exact number of hosts needed without leaving a large pool of unused addresses.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
255.255.255.240
Why it's wrong here
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: answer the scenario, not the keyword
CompTIA A+ often tests the concept that the subnet mask must provide at least the required number of usable hosts, but candidates mistakenly choose a mask that gives exactly the total addresses (e.g., 32) without subtracting the network and broadcast addresses, or they pick a larger subnet (like /26) thinking it is safer, ignoring the 'minimal waste' constraint.
Detailed technical explanation
How to think about this question
The subnet mask 255.255.255.224 corresponds to a /27 prefix length, where the last octet in binary is 11100000, leaving 5 bits for host addresses. The formula for usable hosts is 2^(32 - prefix length) - 2, subtracting the network and broadcast addresses. In real-world deployments, using a /27 for a 30-device branch office avoids IP exhaustion in a larger private address space (e.g., 192.168.1.0/24) while keeping the subnet small enough to limit broadcast traffic.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this 220-1201 question test?
IP Addressing — This question tests IP Addressing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 255.255.255.224 — The network requires exactly 30 usable IP addresses. A /27 subnet (mask 255.255.255.224) provides 32 total addresses, with 30 usable (2^5 - 2 = 30), matching the requirement with minimal waste. This is the most efficient choice because it supplies the exact number of hosts needed without leaving a large pool of unused addresses.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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