CCNA Network Infrastructure and Connectivity Practice Question
What prefix length corresponds to the subnet mask 255.255.255.248?
⚠ Common exam trap
Be careful not to confuse the binary values of subnet masks. Ensure you understand how to convert between dotted-decimal and CIDR notation.
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
✓
/29
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.
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
This is wrong because /28 would correspond to 255.255.255.240.
When this WOULD be correct
In a different question, if asked what prefix length corresponds to a subnet mask of 255.255.255.240, option A would be correct, as that subnet mask corresponds to a /28 prefix length, allowing for 16 IP addresses.
- ✓
/29
Why this is correct
This is correct because 255.255.255.248 equals 29 network bits.
- ✗
/30
Why it's wrong here
This is wrong because /30 would correspond to 255.255.255.252.
When this WOULD be correct
In a different question, if asked for the prefix length of a subnet mask of 255.255.255.252, which corresponds to /30, option C would be correct, as it would accurately reflect the number of usable addresses in that subnet.
- ✗
/27
Why it's wrong here
This is wrong because /27 would correspond to 255.255.255.224.
When this WOULD be correct
In a different question asking for the prefix length of a subnet mask of 255.255.255.224, option D would be correct, as that subnet mask corresponds to a /27 prefix length, allowing for 30 usable 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.
✓/29Correct answer▾
Why this is correct
This is correct because 255.255.255.248 equals 29 network bits.
✗/28Wrong answer — click to see why▾
Why this is wrong here
The subnet mask 255.255.255.240 corresponds to a /28 prefix length, not /29. This mask has 28 network bits, leaving 4 host bits, which yields 14 usable hosts per subnet.
★ When this WOULD be the correct answer
In a different question, if asked what prefix length corresponds to a subnet mask of 255.255.255.240, option A would be correct, as that subnet mask corresponds to a /28 prefix length, allowing for 16 IP addresses.
Why candidates choose this
Students often confuse the prefix lengths for common subnet masks. Since /28 is close to /29, they might mistakenly think that 255.255.255.248 is /28 without calculating the binary representation.
✗/30Wrong answer — click to see why▾
Why this is wrong here
The subnet mask 255.255.255.252 corresponds to a /30 prefix length, not /29. A /30 mask has 30 network bits and only 2 host bits, providing 2 usable addresses, typically used for point-to-point links.
★ When this WOULD be the correct answer
In a different question, if asked for the prefix length of a subnet mask of 255.255.255.252, which corresponds to /30, option C would be correct, as it would accurately reflect the number of usable addresses in that subnet.
Why candidates choose this
Both /29 and /30 are used for small subnets, and students may confuse the number of host bits. Since /30 is a common mask for WAN links, they might incorrectly assume 255.255.255.248 is /30.
✗/27Wrong answer — click to see why▾
Why this is wrong here
The subnet mask 255.255.255.224 corresponds to a /27 prefix length, not /29. A /27 mask has 27 network bits and 5 host bits, providing 30 usable hosts per subnet.
★ When this WOULD be the correct answer
In a different question asking for the prefix length of a subnet mask of 255.255.255.224, option D would be correct, as that subnet mask corresponds to a /27 prefix length, allowing for 30 usable IP addresses.
Why candidates choose this
Students may misremember the binary pattern for /27 and /29. Since both masks have the last octet as a non-standard value, they might incorrectly associate 248 with /27 without proper calculation.
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
Subnetting and VLSM
Key term
Subnetting
Subnetting is the process of dividing a larger IP network into smaller, more manageable subnetworks (subnets) to improve performance, security, and efficient use of IP addresses.
Key term
Subnet mask
A subnet mask is a 32-bit number that helps a computer or network device determine which part of an IP address identifies the network and which part identifies the host device on that network.
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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.