CCNA Network Infrastructure and Connectivity Practice Question
A host uses the subnet mask 255.255.254.0. Which prefix length does this represent?
⚠ Common exam trap
Be cautious of common subnet masks like /24 and ensure you count the bits correctly, especially in non-standard masks.
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
✓
/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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
/22
Why it's wrong here
The /22 prefix length yields a mask of 255.255.252.0, which is binary 11111111.11111111.11111100.00000000. Here the third octet is 252 (11111100), having six consecutive network bits, but the given mask has 254 (11111110) with seven network bits in that octet. Because 254 is numerically one greater than 252, /22 would allow 1024 addresses, not the 512 that 255.255.254.0 provides, so it is incorrect.
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
A prefix length of /23 means 23 bits are set to 1 in the subnet mask, which in binary is 11111111.11111111.11111110.00000000. This converts directly to decimal 255.255.254.0, where the third octet has seven network bits and one host bit. Since the fourth octet contributes eight more host bits, the total host address space is 2^9 = 512 addresses, verifying that /23 is the correct representation.
- ✗
/24
Why it's wrong here
A /24 prefix length corresponds to the subnet mask 255.255.255.0, shown as 11111111.11111111.11111111.00000000. This places all 24 network bits in the first three octets with no borrowing from the fourth octet, resulting in 256 total addresses. In contrast, the given mask 255.255.254.0 uses only 23 bits, leaving one bit in the third octet as a host bit, so /24 is too restrictive and does not match the mask.
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
The /25 prefix length would create a mask of 255.255.255.128, which in binary is 11111111.11111111.11111111.10000000. That mask extends into the fourth octet, using the high-order bit for the network, leaving only 7 bits for hosts and yielding 128 addresses. The provided mask 255.255.254.0 has a zero in the entire fourth octet, so it cannot have 25 network bits; /25 would produce a different fourth-octet value, making it clearly wrong.
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
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓/23Correct answer▾
Why this is correct
A prefix length of /23 means 23 bits are set to 1 in the subnet mask, which in binary is 11111111.11111111.11111110.00000000. This converts directly to decimal 255.255.254.0, where the third octet has seven network bits and one host bit. Since the fourth octet contributes eight more host bits, the total host address space is 2^9 = 512 addresses, verifying that /23 is the correct representation.
✗/22Wrong answer — click to see why▾
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.
✗/24Wrong answer — click to see why▾
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.
✗/25Wrong answer — click to see why▾
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?”
Visual reference
Go deeper
Related to this question
Learn chapter
Subnetting and VLSM
Key term
Classless Inter-domain Routing
Classless Inter-domain Routing (CIDR) is a method for allocating IP addresses and routing IP packets more efficiently by replacing the old class-based system with variable-length subnet masks.
Key term
Prefix length
A prefix length is the number of bits in a subnet mask that identifies the network portion of an IP address, written after a slash (e.g., /24).
About these practice questions
This 200-301 question is part of Courseiva's 1,389-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.