N10-009 Networking Concepts Practice Question
A network engineer needs to create subnets for four departments, each requiring at least 50 host addresses. The available network is 192.168.1.0/24. Which subnet mask should be used to satisfy the requirements while minimizing wasted addresses?
⚠ Common exam trap
Test-takers frequently choose /27 because they calculate 2^5 = 32 and forget to subtract 2 for the network and broadcast addresses, mistakenly thinking 32 hosts are available, or they choose /25 because they see it provides more than 50 hosts without realizing it only creates two subnets, not four.
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
✓
/26 (255.255.255.192)
A /26 subnet mask (255.255.255.192) provides 62 usable host addresses per subnet (2^(32-26) - 2 = 62), which meets the requirement of at least 50 hosts per department. With a /24 network, you can create exactly four /26 subnets (192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/26), perfectly matching the four departments while minimizing wasted addresses.
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 (255.255.255.192)
Why this is correct
The /26 CIDR prefix utilizes 26 bits for the network portion, leaving 6 bits for host addresses. This configuration yields 2^6 = 64 total addresses per subnet, which translates to 62 usable host addresses after accounting for the network and broadcast addresses. When applied to a /24 network, a /26 mask creates 2^(26-24) = 4 distinct subnets, perfectly meeting the requirement for four departments, each needing at least 50 hosts.
- ✗
/25 (255.255.255.128)
Why it's wrong here
A /25 CIDR prefix allocates 25 bits to the network portion, leaving 7 bits for host addresses. This allows for 2^7 = 128 total addresses per subnet, providing 126 usable host addresses after reserving the network and broadcast addresses. Although 126 usable hosts comfortably exceeds the 50-host requirement, a /25 mask only creates 2^(25-24) = 2 subnets from a /24 block, which is insufficient for the specified need of four distinct departmental subnets.
When this WOULD be correct
A network engineer needs to create subnets for two departments, each requiring at least 100 host addresses, using the 192.168.1.0/24 network. A /25 mask would provide two subnets with 126 hosts each, satisfying the requirement.
- ✗
/27 (255.255.255.224)
Why it's wrong here
The /27 CIDR prefix uses 27 bits for the network, leaving only 5 bits for host addresses. This configuration results in 2^5 = 32 total addresses per subnet, which, after subtracting the network and broadcast addresses, provides only 30 usable host addresses. While a /27 mask would create 2^(27-24) = 8 subnets, the individual subnet size of 30 usable hosts is insufficient for departments requiring at least 50 hosts.
When this WOULD be correct
If the requirement were for each subnet to support at least 30 hosts, or if there were 8 departments needing subnets, a /27 mask would be appropriate to minimize waste while meeting the host count.
- ✗
/24 (255.255.255.0)
Why it's wrong here
A /24 CIDR prefix, which is the default mask for a Class C network, designates 24 bits for the network and 8 bits for host addresses. This provides 2^8 = 256 total addresses, resulting in 254 usable hosts within a single network segment. However, this mask does not create multiple subnets; it represents one large broadcast domain, failing to fulfill the requirement for four separate departmental subnets.
When this WOULD be correct
If the requirement were to have a single subnet supporting at least 200 hosts with no need for subnetting, a /24 mask would be correct.
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 N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓/26 (255.255.255.192)Correct answer▾
Why this is correct
The /26 CIDR prefix utilizes 26 bits for the network portion, leaving 6 bits for host addresses. This configuration yields 2^6 = 64 total addresses per subnet, which translates to 62 usable host addresses after accounting for the network and broadcast addresses. When applied to a /24 network, a /26 mask creates 2^(26-24) = 4 distinct subnets, perfectly meeting the requirement for four departments, each needing at least 50 hosts.
✗/25 (255.255.255.128)Wrong answer — click to see why▾
Why this is wrong here
A /25 subnet provides 126 host addresses, which is more than the 50 needed per department, but it only allows for 2 subnets, not the required 4 departments.
★ When this WOULD be the correct answer
A network engineer needs to create subnets for two departments, each requiring at least 100 host addresses, using the 192.168.1.0/24 network. A /25 mask would provide two subnets with 126 hosts each, satisfying the requirement.
Why candidates choose this
Candidates may think a /25 is a common choice for subnetting and might overlook the requirement for four subnets, focusing only on the host count.
✗/27 (255.255.255.224)Wrong answer — click to see why▾
Why this is wrong here
A /27 subnet provides only 30 usable host addresses per subnet (2^(32-27)-2 = 30), which is insufficient for the requirement of at least 50 hosts per department.
★ When this WOULD be the correct answer
If the requirement were for each subnet to support at least 30 hosts, or if there were 8 departments needing subnets, a /27 mask would be appropriate to minimize waste while meeting the host count.
Why candidates choose this
Candidates may mistakenly think /27 provides 62 hosts (confusing with /25) or incorrectly calculate usable addresses as 2^(32-27) = 32 without subtracting the network and broadcast addresses.
✗/24 (255.255.255.0)Wrong answer — click to see why▾
Why this is wrong here
A /24 subnet provides 256 total addresses, which is far more than the 200 needed (4×50), and it cannot be subdivided into four separate subnets without using a different mask.
★ When this WOULD be the correct answer
If the requirement were to have a single subnet supporting at least 200 hosts with no need for subnetting, a /24 mask would be correct.
Why candidates choose this
Candidates may default to the original /24 network mask without considering that subnetting is required to create four separate subnets.
Analysis generated from the official N10-009blueprint 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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.