The answer is 254 usable IP addresses in a /24 subnet. This is correct because a /24 subnet, represented by the subnet mask 255.255.255.0, provides 256 total IP addresses, but two of those are reserved: the network address (the first address, ending in .0) and the broadcast address (the last address, ending in .255), leaving 254 addresses available for devices like computers, printers, and phones. On the CompTIA A+ Core 1 220-1201 exam, this concept tests your understanding of subnetting fundamentals and DHCP allocation, often appearing in scenario-based questions where you must determine if a subnet has enough room for a given number of hosts. A common trap is forgetting to subtract the two reserved addresses, leading to an incorrect count of 256. To remember, think of the “/24 rule”: 256 total, minus 2 reserved, equals 254 usable—plenty for any small office.
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 small office uses a single router with a built-in DHCP server. The office has 15 devices that need IP addresses. The router's default subnet mask is 255.255.255.0. How many usable IP addresses are available for devices on this subnet?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
254
The subnet mask 255.255.255.0 corresponds to a /24 network, which provides 256 total addresses. After reserving the network address and broadcast address, 254 usable IP addresses remain, which is more than enough for 15 devices.
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.
✓
254
Why this is correct
Correct. A /24 network has 256 total addresses minus 2 reserved addresses equals 254 usable addresses.
Incorrect. 255 is the broadcast address value for the last octet, not the count of usable addresses.
✗
256
Why it's wrong here
Incorrect. 256 is the total number of addresses in the subnet, but two are reserved for network and broadcast.
✗
15
Why it's wrong here
Incorrect. 15 is the number of devices in the office, not the number of usable IP addresses in the subnet.
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: 254 — The subnet mask 255.255.255.0 corresponds to a /24 network, which provides 256 total addresses. After reserving the network address and broadcast address, 254 usable IP addresses remain, which is more than enough for 15 devices.
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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