CCNA Network Infrastructure and Connectivity Practice Question
What prefix length corresponds to the subnet mask 255.255.255.224?
⚠ Common exam trap
Be cautious not to confuse the binary values of different subnet masks. Always convert the last octet to binary to determine the correct prefix length.
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
The subnet mask 255.255.255.224 corresponds to /27. In plain language, the first three octets contribute 24 network bits, and 224 in binary is 11100000, which contributes 3 more network bits. That gives a total of 27 network bits. This is a common prefix-conversion question because it checks whether you can move between dotted-decimal masks and prefix lengths confidently.
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
The prefix /26 corresponds to 26 network bits, which produces the subnet mask 255.255.255.192 (binary ...11000000). The mask in question, .224, is ...11100000, representing 27 network bits instead. Additionally, /26 allows 6 host bits and 62 usable addresses, while .224 only permits 5 host bits and 30 usable addresses, so they cannot be equivalent.
When this WOULD be correct
In a different question asking for the prefix length of a subnet mask of 255.255.255.192, option A would be correct as it corresponds to a /26 prefix length. This would involve a scenario where the subnetting needs are for a larger subnet than /27.
- ✓
/27
Why this is correct
The subnet mask 255.255.255.224 converts to binary as 11111111.11111111.11111111.11100000, where the last octet indicates 3 additional network bits beyond the default /24. Adding 24 + 3 gives a prefix length of /27, which is the correct match. This leaves 5 host bits, providing 2^5 - 2 = 30 usable IP addresses per subnet.
- ✗
/28
Why it's wrong here
A /28 prefix means 28 network bits and 4 host bits, corresponding to the subnet mask 255.255.255.240 (binary ...11110000). The given mask .224 is ...11100000, which has 3 host bits and therefore a different boundary. Choosing /28 would imply a larger host range than what the mask actually provides, yielding 14 usable hosts instead of the 30 possible with .224.
When this WOULD be correct
If the question asked for the prefix length corresponding to the subnet mask 255.255.255.240, then option C: /28 would be the correct answer, as it accurately reflects that specific subnet mask.
- ✗
/29
Why it's wrong here
The prefix /29 allocates 29 network bits, leaving only 3 host bits, which produces the subnet mask 255.255.255.248 (binary 11111111.11111111.11111111.11111000). The mask 255.255.255.224 has 5 host bits, not 3, so /29 is too long. This would also limit each subnet to just 6 usable addresses, whereas .224 supports 30 usable addresses.
When this WOULD be correct
In a different exam scenario, if the question asked for the prefix length of a subnet mask of 255.255.255.248, then /29 would be the correct answer, as it provides 8 IP addresses with 6 usable ones.
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
The subnet mask 255.255.255.224 converts to binary as 11111111.11111111.11111111.11100000, where the last octet indicates 3 additional network bits beyond the default /24. Adding 24 + 3 gives a prefix length of /27, which is the correct match. This leaves 5 host bits, providing 2^5 - 2 = 30 usable IP addresses per subnet.
✗/26Wrong answer — click to see why▾
Why this is wrong here
The /26 prefix corresponds to subnet mask 255.255.255.192, which has 64 addresses per subnet, not 32. The mask 255.255.255.224 has 27 network bits, not 26.
★ When this WOULD be the correct answer
In a different question asking for the prefix length of a subnet mask of 255.255.255.192, option A would be correct as it corresponds to a /26 prefix length. This would involve a scenario where the subnetting needs are for a larger subnet than /27.
Why candidates choose this
Students often confuse the number of bits borrowed; /26 is a common prefix and might be mistakenly associated with 224 because both are used in subnetting.
✗/28Wrong answer — click to see why▾
Why this is wrong here
The /28 prefix corresponds to subnet mask 255.255.255.240, which provides 16 addresses per subnet (14 usable). The mask 255.255.255.224 has 32 addresses per subnet, so /28 is incorrect.
★ When this WOULD be the correct answer
If the question asked for the prefix length corresponding to the subnet mask 255.255.255.240, then option C: /28 would be the correct answer, as it accurately reflects that specific subnet mask.
Why candidates choose this
Students may think that 224 in the last octet implies 28 bits because 224 is close to 240, or they might misremember the binary conversion.
✗/29Wrong answer — click to see why▾
Why this is wrong here
The /29 prefix corresponds to subnet mask 255.255.255.248, which provides 8 addresses per subnet (6 usable). The mask 255.255.255.224 has 32 addresses, so /29 is incorrect.
★ When this WOULD be the correct answer
In a different exam scenario, if the question asked for the prefix length of a subnet mask of 255.255.255.248, then /29 would be the correct answer, as it provides 8 IP addresses with 6 usable ones.
Why candidates choose this
Students might confuse the number of host bits; /29 is a common prefix for small subnets, and the 224 mask might be mistakenly associated with a higher prefix length due to its larger numeric value.
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
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
TCP Three-Way Handshake
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using 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).
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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.