CCNA Network Infrastructure and Connectivity Practice Question
A subnet must support 30 usable IPv4 host addresses. Which prefix is the smallest that meets the requirement?
⚠ Common exam trap
A frequent exam trap is choosing a /28 prefix because it appears to be the closest to supporting 30 hosts. However, a /28 subnet only provides 16 total addresses, of which 14 are usable for hosts after excluding the network and broadcast addresses. This mistake arises from confusing total addresses with usable hosts or failing to subtract the reserved addresses. Selecting a /28 leads to insufficient host capacity, causing network issues or exam failure. Always remember that usable hosts equal total addresses minus two, which is critical when calculating subnet sizes for CCNA questions.
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
✓
/27
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
/28
Why it's wrong here
A /28 prefix, with a 255.255.255.240 mask, provides only 16 total addresses. Deducting the network and broadcast addresses leaves just 14 usable host addresses, which falls far short of the required 30. Consequently, /28 cannot be used because it simply does not have enough host bits to address the necessary number of devices.
When this WOULD be correct
If the question specified a requirement for 14 usable IPv4 host addresses instead of 30, then a /28 subnet would be the correct answer, as it meets that specific need.
- ✓
/27
Why this is correct
A /27 prefix has a subnet mask of 255.255.255.224, which creates 32 total addresses. After reserving the network address and broadcast address, exactly 30 usable host addresses remain, making it the smallest prefix that satisfies the requirement of 30 usable IPv4 hosts. Thus, /27 is the correct choice because it provides the precise number of needed addresses without wasting space.
- ✗
/26
Why it's wrong here
A /26 prefix uses a 255.255.255.192 mask and yields 64 total addresses, of which 62 are usable for hosts. While this technically supports 30 usable addresses and would function, it allocates twice the necessary address space. The requirement asks for the prefix that must support 30 usable hosts, implying the most efficient or minimal prefix; /26 is larger than needed, so it is not the correct answer.
When this WOULD be correct
If the exam question required a subnet that supports at least 62 usable addresses, then /26 would be the correct answer, as it provides the necessary capacity while still being a valid subnet size.
- ✗
/25
Why it's wrong here
A /25 prefix has a 255.255.255.128 mask, giving 128 total addresses and 126 usable hosts. Although it comfortably exceeds the 30-host requirement, it consumes eight times the address space of a /27 and represents a significant waste of IPv4 address space. In a well-designed addressing plan, you should choose the smallest prefix that meets the host count, so /25 is incorrect because it is vastly over-provisioned.
When this WOULD be correct
If the exam question specified a requirement for a subnet that supports up to 126 usable addresses, then /25 would be the correct choice, as it would provide sufficient capacity for that scenario.
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.
✓/27Correct answer▾
Why this is correct
A /27 prefix has a subnet mask of 255.255.255.224, which creates 32 total addresses. After reserving the network address and broadcast address, exactly 30 usable host addresses remain, making it the smallest prefix that satisfies the requirement of 30 usable IPv4 hosts. Thus, /27 is the correct choice because it provides the precise number of needed addresses without wasting space.
✗/28Wrong answer — click to see why▾
Why this is wrong here
A /28 subnet provides only 14 usable host addresses (16 total minus 2 for network and broadcast), which is insufficient for the requirement of 30 usable addresses.
★ When this WOULD be the correct answer
If the question specified a requirement for 14 usable IPv4 host addresses instead of 30, then a /28 subnet would be the correct answer, as it meets that specific need.
Why candidates choose this
Candidates may choose /28 due to a misunderstanding of subnetting calculations, mistakenly believing that a smaller subnet can still accommodate more hosts than it actually can.
✗/26Wrong answer — click to see why▾
Why this is wrong here
Option C: /26 provides 62 usable addresses, which exceeds the requirement of 30 usable addresses. Therefore, it is not the smallest prefix that meets the specified need.
★ When this WOULD be the correct answer
If the exam question required a subnet that supports at least 62 usable addresses, then /26 would be the correct answer, as it provides the necessary capacity while still being a valid subnet size.
Why candidates choose this
Candidates may choose /26 due to a misunderstanding of subnetting, mistakenly believing that a larger subnet is always acceptable without considering the requirement for the smallest prefix.
✗/25Wrong answer — click to see why▾
Why this is wrong here
Option D: /25 provides 126 usable addresses, which exceeds the requirement of 30 usable addresses. Therefore, it is not the smallest prefix that meets the specified need.
★ When this WOULD be the correct answer
If the exam question specified a requirement for a subnet that supports up to 126 usable addresses, then /25 would be the correct choice, as it would provide sufficient capacity for that scenario.
Why candidates choose this
Candidates may choose /25 due to a misunderstanding of subnetting, thinking that a larger subnet is always acceptable, or they might miscalculate the number of usable addresses needed for the given requirement.
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
Usable hosts
Usable hosts are the IP addresses in a subnet that can actually be assigned to devices like computers, printers, or servers, excluding the network and broadcast addresses.
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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.