Quick Answer
The answer is /23. This prefix length is derived by converting the subnet mask 255.255.254.0 into binary: the first two octets are all ones (16 bits), the third octet 254 in binary is 11111110 (7 ones), and the last octet is all zeros (0 bits), giving a total of 16 + 7 = 23 network bits. On the CCNA 200-301 v2 exam, this type of subnet mask to prefix length conversion tests your ability to move beyond memorizing common /24 or /30 masks and handle non-octet boundaries, which frequently appear in subnetting questions and local-scope calculations. A common trap is assuming that 255.255.254.0 equals /24 because the third octet is close to 255, but the missing final bit reduces the prefix by one. A quick memory tip: think of 254 as “one less than 255,” meaning you lose one network bit from the full octet, so a /24 becomes a /23.
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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. A key principle to apply: subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.. 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 host uses the subnet mask 255.255.254.0. Which prefix length does this represent?
/22
Why wrong: This is wrong because /22 would correspond to 255.255.252.0.
/23
This is correct because 255.255.254.0 corresponds to 23 network bits.
/24
Why wrong: This is wrong because /24 would correspond to 255.255.255.0.
/25
Why wrong: This is wrong because /25 would correspond to 255.255.255.128.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
/23
The mask 255.255.254.0 represents /23. In plain language, the first two octets are fully network bits, the third octet contributes 7 more network bits because 254 in binary is 11111110, and the last octet contributes none. That totals 23 network bits. This is a common prefix-conversion question because candidates often memorize the usual masks but hesitate when a non-/24 boundary appears. Recognizing that 255.255.254.0 equals /23 is important for subnetting and local-scope calculations.
Key principle: Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
/22
Why it's wrong here
This is wrong because /22 would correspond to 255.255.252.0.
When this WOULD be correct
In a different question asking for the prefix length of a subnet mask of 255.255.252.0, option A would be correct, as that mask represents a /22 prefix length.
/23
Why this is correct
This is correct because 255.255.254.0 corresponds to 23 network bits.
Related concept
Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.
/24
Why it's wrong here
This is wrong because /24 would correspond to 255.255.255.0.
When this WOULD be correct
If the question asked for the prefix length of a subnet mask of 255.255.255.0, then option C (/24) would be correct, as this mask allows for 256 IP addresses in a single subnet, suitable for smaller networks.
/25
Why it's wrong here
This is wrong because /25 would correspond to 255.255.255.128.
When this WOULD be correct
If the question asked for the prefix length of a subnet mask of 255.255.255.128, then option D would be correct, as that subnet mask corresponds to a /25 prefix length, allowing for 128 IP addresses.
Option-by-option analysis
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
Why this is correct
This is correct because 255.255.254.0 corresponds to 23 network bits.
Why this is wrong here
Option A is incorrect because the subnet mask 255.255.254.0 corresponds to a prefix length of /23, not /22. A /22 prefix would indicate a subnet mask of 255.255.252.0.
★ When this WOULD be the correct answer
In a different question asking for the prefix length of a subnet mask of 255.255.252.0, option A would be correct, as that mask represents a /22 prefix length.
Why candidates choose this
Candidates may choose this option due to confusion between the number of bits used for the subnet mask and the resulting prefix length, leading them to miscalculate the correct prefix.
Why this is wrong here
The subnet mask 255.255.254.0 corresponds to a prefix length of /23, not /24. A /24 prefix length would indicate a subnet mask of 255.255.255.0, which allows for fewer hosts per subnet.
★ When this WOULD be the correct answer
If the question asked for the prefix length of a subnet mask of 255.255.255.0, then option C (/24) would be correct, as this mask allows for 256 IP addresses in a single subnet, suitable for smaller networks.
Why candidates choose this
Candidates may choose /24 because it is a common subnet mask for smaller networks, leading to confusion when interpreting subnet masks and their corresponding prefix lengths.
Why this is wrong here
Option D is incorrect because a subnet mask of 255.255.254.0 corresponds to a prefix length of /23, not /25. A /25 subnet mask would allow for only 128 IP addresses, which is not applicable here.
★ When this WOULD be the correct answer
If the question asked for the prefix length of a subnet mask of 255.255.255.128, then option D would be correct, as that subnet mask corresponds to a /25 prefix length, allowing for 128 IP addresses.
Why candidates choose this
Candidates might choose option D due to confusion between the number of available hosts and the prefix length, as they may associate smaller subnet sizes with higher prefix lengths without fully understanding the relationship between subnet masks and their corresponding prefix lengths.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Be cautious of common subnet masks like /24 and ensure you count the bits correctly, especially in non-standard masks.
Detailed technical explanation
Subnetting is the process of dividing a larger IP network into smaller, more manageable subnetworks by manipulating the subnet mask or prefix length. The subnet mask 255.255.254.0 is a non-standard mask that extends beyond the common /24 boundary, indicating that the network portion spans more bits than a typical Class C network. This mask means the first two octets (255.255) are fully network bits, the third octet (254) contributes 7 bits, and the last octet (0) contributes none, totaling 23 bits for the network prefix. The prefix length, also called CIDR notation, counts the number of bits set to 1 in the subnet mask. For 255.255.254.0, the binary form is 11111111.11111111.11111110.00000000, which equals /23. This means the first 23 bits identify the network, and the remaining 9 bits identify hosts within that subnet. Cisco devices use this prefix length to determine routing boundaries, broadcast domains, and address allocation. A common exam trap is confusing 255.255.254.0 with masks like 255.255.252.0 (/22) or 255.255.255.0 (/24). Candidates often default to /24 because it is a common subnet size, but the presence of 254 in the third octet means one bit fewer than 255, indicating a /23 prefix. Understanding this distinction is critical for accurate subnetting, IP address planning, and configuring Cisco routers or switches correctly in the CCNA exam context.
Key takeaway
Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.
Real-world example
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.
What to study next
Review subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host..
The correct answer is: /23 — The mask 255.255.254.0 represents /23. In plain language, the first two octets are fully network bits, the third octet contributes 7 more network bits because 254 in binary is 11111110, and the last octet contributes none. That totals 23 network bits. This is a common prefix-conversion question because candidates often memorize the usual masks but hesitate when a non-/24 boundary appears. Recognizing that 255.255.254.0 equals /23 is important for subnetting and local-scope calculations.
Review subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host., then practise related 200-301 questions on the same topic to reinforce the concept.
Subnet masks define the boundary between network and host portions of an IP address by setting bits to 1 for network and 0 for host.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. What prefix length corresponds to the subnet mask 255.255.255.248?
hardWhy B: The mask 255.255.255.248 corresponds to /29. In practical terms, the first three octets provide 24 network bits, and the value 248 in the last octet is 11111000 in binary, which contributes 5 more network bits. That gives a total prefix length of 29. This is a standard dotted-decimal to prefix conversion question. It matters because subnetting often requires you to move comfortably between both forms.
Variation 2. What prefix length corresponds to the subnet mask 255.255.255.224?
hardWhy B: The subnet mask 255.255.255.224 corresponds to /27. In plain language, the first three octets contribute 24 network bits, and 224 in binary is 11100000, which contributes 3 more network bits. That gives a total of 27 network bits. This is a common prefix-conversion question because it checks whether you can move between dotted-decimal masks and prefix lengths confidently.
Variation 3. Which prefix length corresponds to the subnet mask 255.255.255.192?
hardWhy B: The mask 255.255.255.192 corresponds to /26. In practical terms, the first three octets contribute 24 network bits, and 192 in binary is 11000000, which contributes 2 more network bits. That totals 26 network bits. This is a standard conversion skill that matters in subnetting, ACL design, and route interpretation.
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Last reviewed: May 17, 2026
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