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A network engineer needs to assign IP addresses to a new subnet that will support exactly 25 devices. Which subnet mask would provide the minimum number of usable host addresses while still accommodating the requirement?

Question 1mediummultiple choice
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A network engineer needs to assign IP addresses to a new subnet that will support exactly 25 devices. Which subnet mask would provide the minimum number of usable host addresses while still accommodating the requirement?

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Answer choices

Why each option matters

Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.

A

Distractor review

255.255.255.240 (/28)

/28 provides 14 usable hosts, which is insufficient for 25 devices.

B

Best answer

255.255.255.224 (/27)

/27 provides 28 usable hosts, which meets the requirement with minimal waste.

C

Distractor review

255.255.255.192 (/26)

/26 provides 62 usable hosts, which is more than needed but not the minimum.

D

Distractor review

255.255.255.128 (/25)

/25 provides 126 usable hosts, far exceeding the need and not the minimal subnet.

Common exam trap

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.

Technical deep dive

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.

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More questions from this exam

Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.

Question 1

A network engineer needs to connect two switches located 400 meters apart. The cable run includes high electromagnetic interference from nearby machinery. The engineer decides to use fiber optic cabling. Which transceiver type and fiber combination should be used to ensure the link reaches 400 meters while remaining cost-effective?

Question 2

A network engineer is designing a new switched network and needs to ensure that broadcast traffic from one department does not reach another department's workstations. The engineer plans to use VLANs. Which of the following must be configured on the switches to isolate broadcast domains as intended?

Question 3

A security engineer is configuring a site-to-site VPN between two branch offices. The requirement is to encrypt all traffic between the two networks using IPsec. Which IPsec mode should be used to encrypt the entire IP packet including the original header?

Question 4

A network administrator is connecting two switches to increase bandwidth and provide redundancy. Which technology should be used to combine multiple physical links into a single logical link?

Question 5

A network administrator is experiencing issues where unauthorized devices are offering IP addresses to clients, causing connectivity problems. Which security feature should be enabled on switches to prevent this?

Question 6

A network administrator is troubleshooting a connectivity issue and suspects the problem is related to the physical cabling. At which layer of the OSI model should the administrator begin their investigation?

FAQ

Questions learners often ask

What does this N10-009 question test?

CIDR notation defines the prefix length.

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 30 total addresses per subnet, with 30 - 2 = 28 usable host addresses. This is the smallest subnet that can support at least 25 hosts. /28 gives 14 usable hosts (too few), /26 gives 62 usable hosts (more than needed but not minimal), and /25 gives 126 usable hosts.

What should I do if I get this N10-009 question wrong?

Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.

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