- A
254
Correct. A /24 network has 256 total addresses minus 2 reserved addresses equals 254 usable addresses.
- B
255
Why wrong: Incorrect. 255 is the broadcast address value for the last octet, not the count of usable addresses.
- C
256
Why wrong: Incorrect. 256 is the total number of addresses in the subnet, but two are reserved for network and broadcast.
- D
15
Why wrong: Incorrect. 15 is the number of devices in the office, not the number of usable IP addresses in the subnet.
Question 819 of 1,020
How Many Usable IP Addresses in a /24 Subnet?
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?
Quick Answer
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.
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
✓
254
The subnet mask 255.255.255.0 corresponds to a /24 prefix, which provides 256 total addresses (2^8). However, the first address (network address) and the last address (broadcast address) are reserved, leaving 254 usable IP addresses for devices. This is a fundamental IPv4 subnetting rule defined in RFC 791.
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.
- ✓
254
Why this is correct
Correct. A /24 network has 256 total addresses minus 2 reserved addresses equals 254 usable addresses.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
255
Why it's wrong here
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: answer the scenario, not the keyword
Cisco often tests the distinction between total addresses and usable addresses, trapping candidates who forget to subtract the network and broadcast addresses, leading them to choose 256 instead of 254.
Detailed technical explanation
How to think about this question
In a /24 subnet, the network address is the first IP (e.g., 192.168.1.0) and the broadcast address is the last IP (e.g., 192.168.1.255). The usable range is 192.168.1.1 through 192.168.1.254. Even if only 15 devices are present, the subnet still offers 254 usable addresses; the DHCP server can assign from this pool, and the router's default lease time (often 24 hours) does not affect the total count.
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
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
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: 254 — The subnet mask 255.255.255.0 corresponds to a /24 prefix, which provides 256 total addresses (2^8). However, the first address (network address) and the last address (broadcast address) are reserved, leaving 254 usable IP addresses for devices. This is a fundamental IPv4 subnetting rule defined in RFC 791.
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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