The correct subnet mask for 20 hosts is 255.255.255.224, which corresponds to a /27 prefix length. This works because a /27 subnet provides 32 total IP addresses, with 30 usable addresses after subtracting the network and broadcast addresses (2^5 - 2 = 30), making it the most efficient choice that comfortably accommodates 20 devices while minimizing wasted addresses. On the CompTIA Network+ N10-009 exam, this type of question tests your ability to match subnet sizes to host requirements, often appearing in VLAN or network segmentation scenarios. A common trap is choosing a /28, which offers only 14 usable addresses—insufficient for 20 hosts—or a /26, which wastes too many addresses. To remember the /27 mask, think of the 224 in the last octet: it means you borrow 3 bits for the network (128+64+32=224), leaving 5 bits for hosts, which gives exactly 30 usable IPs—just right for a small workgroup.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. 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 network administrator is creating a new VLAN that will contain 20 devices. The administrator wants to use the most efficient subnet that provides enough usable IP addresses while minimizing waste. Which of the following subnet masks should be used?
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 (/27)
A /27 subnet mask (255.255.255.224) provides 32 total addresses, of which 30 are usable (2^5 - 2 = 30). This is the most efficient choice for 20 devices because it offers exactly enough usable IPs with minimal waste (only 10 unused addresses), whereas a /28 would provide only 14 usable addresses (insufficient) and a /29 would provide only 6 usable addresses (also insufficient).
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.
✗
255.255.255.240 (/28)
Why it's wrong here
A /28 provides only 14 usable addresses, insufficient for 20 hosts.
✓
255.255.255.224 (/27)
Why this is correct
A /27 provides 30 usable addresses, which is the smallest subnet that can accommodate 20 hosts.
Related concept
Read the scenario before looking for a memorised answer.
✗
255.255.255.248 (/29)
Why it's wrong here
A /29 provides only 6 usable addresses, far too few.
✗
255.255.255.192 (/26)
Why it's wrong here
A /26 provides 62 usable addresses, which is more than needed and wastes address space.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the total number of addresses in a subnet with the number of usable host addresses, forgetting to subtract 2 for the network and broadcast addresses, and may incorrectly select a /28 thinking it provides 16 addresses (when only 14 are usable).
Detailed technical explanation
How to think about this question
The subnet mask determines the number of host bits: a /27 leaves 5 host bits, yielding 2^5 = 32 total addresses per subnet, with the first address reserved as the network ID and the last as the broadcast address. In real-world deployment, administrators often round up to the next power of two to accommodate future growth, but the question explicitly asks for the most efficient subnet that minimizes waste while meeting the current requirement of 20 devices.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Implementation — This question tests Network Implementation — 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 (/27) — A /27 subnet mask (255.255.255.224) provides 32 total addresses, of which 30 are usable (2^5 - 2 = 30). This is the most efficient choice for 20 devices because it offers exactly enough usable IPs with minimal waste (only 10 unused addresses), whereas a /28 would provide only 14 usable addresses (insufficient) and a /29 would provide only 6 usable addresses (also insufficient).
What should I do if I get this N10-009 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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Question Discussion
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