Question 70 of 464
N10-009 Network Implementation Practice Question
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?
⚠ Common exam trap
Test-takers frequently 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).
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 (/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).
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 subnet mask (255.255.255.240) provides 4 host bits (32-28). Consequently, it supports 2^4 = 16 total IP addresses. When subtracting the network and broadcast addresses, only 14 usable host addresses remain. This quantity is insufficient to assign an IP address to each of the 20 devices in the new VLAN, making this option inadequate.
When this WOULD be correct
This subnet mask would be correct for a VLAN that needs exactly 14 usable IP addresses, such as a small management or printer VLAN with 14 devices.
- ✓
255.255.255.224 (/27)
Why this is correct
This subnet mask, corresponding to a /27 CIDR prefix, allocates 5 host bits (32-27). This allows for 2^5 = 32 total IP addresses within the subnet. After reserving the network address and the broadcast address, there are 30 usable host addresses available. This is the smallest and most efficient subnet size that can accommodate the required 20 devices without significant waste of IP address space.
- ✗
255.255.255.248 (/29)
Why it's wrong here
The /29 subnet mask (255.255.255.248) designates 3 host bits (32-29). This configuration yields 2^3 = 8 total IP addresses. After accounting for the mandatory network and broadcast addresses, only 6 usable IP addresses are available for hosts. This is a severe shortfall for a VLAN requiring 20 devices and is typically used for point-to-point links or very small, specialized subnets.
When this WOULD be correct
This subnet mask would be correct for a VLAN requiring exactly 6 usable addresses, such as a point-to-point link or a small management network with only a few devices.
- ✗
255.255.255.192 (/26)
Why it's wrong here
A /26 subnet mask (255.255.255.192) provides 6 host bits (32-26). This results in 2^6 = 64 total IP addresses within the subnet. After reserving the network and broadcast addresses, 62 usable host addresses are available. While this subnet can certainly accommodate 20 devices, it allocates significantly more addresses than necessary, leading to inefficient use of IP address space and potential resource depletion in larger network designs.
When this WOULD be correct
This subnet mask would be correct if the VLAN needed to support 50 devices, as a /26 provides 62 usable addresses, which is the smallest subnet that can accommodate 50 hosts.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓255.255.255.224 (/27)Correct answer▾
Why this is correct
This subnet mask, corresponding to a /27 CIDR prefix, allocates 5 host bits (32-27). This allows for 2^5 = 32 total IP addresses within the subnet. After reserving the network address and the broadcast address, there are 30 usable host addresses available. This is the smallest and most efficient subnet size that can accommodate the required 20 devices without significant waste of IP address space.
✗255.255.255.240 (/28)Wrong answer — click to see why▾
Why this is wrong here
A /28 subnet provides only 14 usable IP addresses (2^4 - 2 = 14), which is insufficient for 20 devices.
★ When this WOULD be the correct answer
This subnet mask would be correct for a VLAN that needs exactly 14 usable IP addresses, such as a small management or printer VLAN with 14 devices.
Why candidates choose this
Candidates may mistakenly think 16 total addresses (2^4) are enough for 20 devices, or they may confuse usable addresses with total addresses.
✗255.255.255.248 (/29)Wrong answer — click to see why▾
Why this is wrong here
A /29 subnet provides only 6 usable IP addresses (2^(32-29)-2 = 6), which is insufficient for 20 devices.
★ When this WOULD be the correct answer
This subnet mask would be correct for a VLAN requiring exactly 6 usable addresses, such as a point-to-point link or a small management network with only a few devices.
Why candidates choose this
Candidates may mistakenly think a /29 is sufficient because they forget to subtract the network and broadcast addresses, or they misapply the formula for usable hosts.
✗255.255.255.192 (/26)Wrong answer — click to see why▾
Why this is wrong here
A /26 subnet provides 62 usable addresses, which is excessive for 20 devices and wastes IP space. The question asks for the most efficient subnet with minimal waste, so /27 (30 usable addresses) is better.
★ When this WOULD be the correct answer
This subnet mask would be correct if the VLAN needed to support 50 devices, as a /26 provides 62 usable addresses, which is the smallest subnet that can accommodate 50 hosts.
Why candidates choose this
Candidates may mistakenly think a larger subnet is safer or fail to calculate the exact number of usable addresses needed, leading them to choose a subnet with more capacity than necessary.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Last reviewed: Jun 11, 2026
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