CCNA Network Infrastructure and Connectivity Practice Question
A subnet must support at least 126 usable IPv4 host addresses. Which prefix is the longest that meets the requirement?
⚠ Common exam trap
Be careful not to confuse total addresses with usable addresses. Always subtract the network and broadcast addresses when calculating usable hosts.
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
✓
/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.
Answer analysis
Option-by-option breakdown
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.
- ✗
/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
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.
✓/25Correct answer▾
Why this is correct
This is correct because /25 provides 126 usable host addresses.
✗/26Wrong answer — click to see why▾
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.
✗/24Wrong answer — click to see why▾
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.
✗/27Wrong answer — click to see why▾
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?”
Visual reference
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Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
IPv4
IPv4 is the fourth version of the Internet Protocol, a set of rules that assigns unique numerical addresses to devices so they can communicate over networks like the internet.
Key term
Internet Protocol
Internet Protocol (IP) is the set of rules that governs how data is addressed, routed, and sent from one device to another across networks, including the internet.
About these practice questions
Courseiva writes every 200-301 question from scratch — 1,389 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or 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.