Quick Answer
The answer is /25. This is the correct choice because to calculate the minimum subnet prefix for usable hosts, you must find the smallest subnet that provides at least 126 usable addresses after reserving the network and broadcast addresses. A /25 prefix yields 128 total IPv4 addresses, leaving exactly 126 usable hosts, while a /26 would only support 62 usable hosts and thus fails the requirement. On the CCNA 200-301 v2 exam, this type of question tests your ability to apply the formula 2^(32-prefix) - 2 to quickly determine usable host counts, often appearing in subnetting scenarios where you must avoid wasting IP space. A common trap is forgetting to subtract the two reserved addresses, leading to an incorrect /24 choice. Memory tip: think of the “host bit count” — for every bit you borrow, you halve the usable hosts; a /25 gives you 7 host bits (2^7 = 128, minus 2 = 126), so remember “7 bits for 126 hosts.”
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: subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.. 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 subnet must support at least 126 usable IPv4 host addresses. Which prefix is the longest that meets the requirement?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
/26
Why wrong: This is wrong because /26 provides only 62 usable host addresses.
/25
This is correct because /25 provides 126 usable host addresses.
/24
Why wrong: This is wrong because /24 works but is larger than necessary.
/27
Why wrong: This is wrong because /27 provides only 30 usable hosts.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
/25
A /25 is the smallest valid answer. In practical terms, a /25 provides 128 total addresses. After subtracting the network and broadcast addresses, 126 usable hosts remain. A /26 would be too small because it supports only 62 usable hosts. This is a typical minimum-prefix question. The goal is to choose the smallest subnet that satisfies the host requirement without wasting more address space than necessary.
Key principle: Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
/26
Why it's wrong here
This is wrong because /26 provides only 62 usable host addresses.
When this WOULD be correct
In a different question, if the requirement was for a subnet that supports at least 62 usable addresses, then /26 would be the correct answer, as it meets that specific need.
/25
Why this is correct
This is correct because /25 provides 126 usable host addresses.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.
/24
Why it's wrong here
This is wrong because /24 works but is larger than necessary.
When this WOULD be correct
If the question specified a need for at least 254 usable addresses, then /24 would be the correct answer, as it provides the required number of usable addresses while being the smallest prefix that meets that specific need.
/27
Why it's wrong here
This is wrong because /27 provides only 30 usable hosts.
When this WOULD be correct
If the question specified a requirement for a smaller network, such as needing to support 30 usable addresses for a small office, then /27 would be the correct answer, as it would meet that specific need.
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 /25 provides 126 usable host addresses.
Why this is wrong here
A /26 prefix provides 2^(32-26) - 2 = 64 - 2 = 62 usable host addresses, which is insufficient for the requirement of at least 126 usable hosts.
★ When this WOULD be the correct answer
In a different question, if the requirement was for a subnet that supports at least 62 usable addresses, then /26 would be the correct answer, as it meets that specific need.
Why candidates choose this
Students often confuse the number of usable hosts with total addresses, forgetting to subtract the network and broadcast addresses, or they may misremember the host formula.
Why this is wrong here
A /24 prefix provides 2^(32-24) - 2 = 256 - 2 = 254 usable host addresses, which is more than required. While it works, it is not the smallest prefix that meets the requirement, wasting IP address space.
★ When this WOULD be the correct answer
If the question specified a need for at least 254 usable addresses, then /24 would be the correct answer, as it provides the required number of usable addresses while being the smallest prefix that meets that specific need.
Why candidates choose this
Students may default to /24 as a common subnet size without calculating the exact requirement, or they may think larger is always better without considering efficiency.
Why this is wrong here
A /27 prefix provides 2^(32-27) - 2 = 32 - 2 = 30 usable host addresses, far below the required 126. This is insufficient for the subnet.
★ When this WOULD be the correct answer
If the question specified a requirement for a smaller network, such as needing to support 30 usable addresses for a small office, then /27 would be the correct answer, as it would meet that specific need.
Why candidates choose this
Students might mistakenly think /27 provides more hosts than it does, or they may confuse the prefix length with the number of hosts, e.g., thinking /27 gives 27 usable addresses.
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 careful not to confuse total addresses with usable addresses. Always subtract the network and broadcast addresses when calculating usable hosts.
Detailed technical explanation
Subnetting is a fundamental concept in IPv4 networking that divides a larger network into smaller, manageable segments called subnets. Each subnet has a network prefix that determines the number of available IP addresses. The prefix length, expressed as a slash followed by a number (e.g., /25), indicates how many bits are fixed for the network portion, with the remaining bits available for host addressing. Usable host addresses exclude the network and broadcast addresses, which are reserved for subnet identification and broadcast traffic respectively. To determine the smallest subnet prefix that supports at least 126 usable hosts, you calculate the number of host bits needed. A /25 prefix leaves 7 bits for hosts (32 - 25 = 7), providing 2^7 = 128 total addresses. Subtracting 2 for network and broadcast addresses yields 126 usable hosts, exactly meeting the requirement. Prefixes longer than /25 (like /26 or /27) provide fewer usable hosts and thus do not meet the minimum requirement, while shorter prefixes (like /24) provide more addresses but are unnecessarily large. A common exam trap is selecting a /24 prefix because it is a familiar subnet size with 254 usable hosts, which exceeds the requirement but wastes address space. Conversely, choosing a /26 or /27 prefix is tempting due to their common usage but results in insufficient host capacity. In practical Cisco networking, efficient subnetting conserves IP address space and optimizes routing tables, making the /25 subnet the precise and efficient choice for exactly 126 hosts.
Key takeaway
Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.
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 subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing., 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 — Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing..
The correct answer is: /25 — A /25 is the smallest valid answer. In practical terms, a /25 provides 128 total addresses. After subtracting the network and broadcast addresses, 126 usable hosts remain. A /26 would be too small because it supports only 62 usable hosts. This is a typical minimum-prefix question. The goal is to choose the smallest subnet that satisfies the host requirement without wasting more address space than necessary.
Review subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing., then practise related 200-301 questions on the same topic to reinforce the concept.
Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
Subnetting divides an IPv4 network into smaller subnets by extending the network prefix to allocate bits for host addressing.
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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. A network requires at least 500 usable host addresses in one IPv4 subnet. Which prefix is the smallest that meets the requirement?
hardWhy B: To support at least 500 usable hosts, the subnet must provide at least 502 total addresses when the network and broadcast addresses are included. In plain language, that means 256 total addresses in a /24 are not enough, so the next larger power-of-two block is required. A /23 provides 512 total addresses and 510 usable host addresses, which satisfies the requirement while remaining the smallest valid option. This is a classic host-capacity question because it checks whether you can work backward from a required usable host count and choose the smallest prefix that works without wasting more space than necessary.
Variation 2. A subnet must support 30 usable IPv4 host addresses. Which prefix is the smallest that meets the requirement?
hardWhy B: A /27 is the smallest valid prefix. In plain language, a /27 provides 32 total addresses, and after subtracting the network and broadcast addresses, 30 usable host addresses remain. A /28 would be too small because it provides only 14 usable hosts. This is a classic minimum-prefix question because it checks whether you can work backward from a host requirement and choose the smallest subnet that fits without over-allocating more space than necessary.
Variation 3. A subnet requires 200 usable host addresses. Which prefix is the smallest that meets the requirement?
hardWhy B: A /24 is the smallest valid choice. In plain language, the subnet needs enough total addresses so that after the network and broadcast addresses are reserved, 200 hosts still remain. A /25 is too small because it provides only 126 usable hosts. A /24 provides 254 usable hosts, which satisfies the requirement while being the next logical prefix size up. This is a standard host-capacity planning question. The key is to work from usable hosts, not just total addresses, and then choose the smallest prefix that actually works.
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Last reviewed: May 17, 2026
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