N10-009 Networking Concepts Practice Question
A network has a single switch with VLANs 10, 20, and 30 configured. The switch is connected to a router that has three subinterfaces, each in a different VLAN. How many broadcast domains are present?
⚠ Common exam trap
The trap here is that candidates often count the router subinterfaces as separate broadcast domains, not realizing that broadcast domains are strictly Layer 2 constructs and that the router only provides inter-VLAN routing without adding new broadcast domains.
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
✓
3
Each VLAN is a separate Layer 2 broadcast domain. With VLANs 10, 20, and 30 configured on the switch and a router using subinterfaces to route between them, there are exactly three broadcast domains — one per VLAN. Broadcasts are confined to their VLAN and do not cross VLAN boundaries without a Layer 3 device.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
1
Why it's wrong here
A single broadcast domain would imply that all devices on the network, regardless of their VLAN assignment, would receive every broadcast frame. However, the fundamental purpose of VLANs is to segment a network into multiple logical broadcast domains, preventing broadcasts from one VLAN from reaching another. Since VLANs 10, 20, and 30 are explicitly configured, they inherently establish three distinct broadcast boundaries, not just one.
When this WOULD be correct
If the question described a single flat network with no VLANs (e.g., all devices on the same switch without VLAN configuration) and no router segmentation, then there would be only one broadcast domain.
- ✓
3
Why this is correct
A broadcast domain is a network segment where all devices can receive each other's broadcast frames. By definition, each Virtual Local Area Network (VLAN) isolates broadcast traffic to its own members, effectively creating a distinct broadcast domain. With VLANs 10, 20, and 30 configured on the switch, there are precisely three separate broadcast domains, as the router's subinterfaces facilitate inter-VLAN routing but do not merge these isolated domains.
- ✗
4
Why it's wrong here
The number of broadcast domains directly corresponds to the number of distinct VLANs configured on the switch. Each VLAN, by its very nature, forms an isolated broadcast domain. Since the problem statement explicitly mentions only three VLANs (10, 20, and 30), there is no basis for a fourth broadcast domain without an additional, unmentioned VLAN or a separate physical network segment.
When this WOULD be correct
If the question stated that the switch had four VLANs (e.g., VLANs 10, 20, 30, and 40) and each was connected to a router subinterface, then there would be four broadcast domains, making option C correct.
- ✗
5
Why it's wrong here
A router's primary function is to forward traffic between different broadcast domains, not to create new ones itself in this context. While a router might have multiple interfaces, each connected to a distinct network segment or VLAN, it acts as a boundary for broadcasts. Therefore, configuring router subinterfaces for inter-VLAN routing does not introduce additional broadcast domains beyond the three already established by VLANs 10, 20, and 30.
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.
✓3Correct answer▾
Why this is correct
A broadcast domain is a network segment where all devices can receive each other's broadcast frames. By definition, each Virtual Local Area Network (VLAN) isolates broadcast traffic to its own members, effectively creating a distinct broadcast domain. With VLANs 10, 20, and 30 configured on the switch, there are precisely three separate broadcast domains, as the router's subinterfaces facilitate inter-VLAN routing but do not merge these isolated domains.
✗1Wrong answer — click to see why▾
Why this is wrong here
Each VLAN creates a separate broadcast domain. With three VLANs (10, 20, 30) and a router with three subinterfaces, there are three distinct broadcast domains, not one.
★ When this WOULD be the correct answer
If the question described a single flat network with no VLANs (e.g., all devices on the same switch without VLAN configuration) and no router segmentation, then there would be only one broadcast domain.
Why candidates choose this
Candidates may mistakenly think that a single switch inherently creates one broadcast domain, ignoring that VLANs logically separate broadcast traffic.
✗4Wrong answer — click to see why▾
Why this is wrong here
Each VLAN creates its own broadcast domain, and with three VLANs (10, 20, 30) there are exactly three broadcast domains. Option C (4) is incorrect because there is no fourth broadcast domain; the router subinterfaces do not add additional broadcast domains beyond the VLANs.
★ When this WOULD be the correct answer
If the question stated that the switch had four VLANs (e.g., VLANs 10, 20, 30, and 40) and each was connected to a router subinterface, then there would be four broadcast domains, making option C correct.
Why candidates choose this
Candidates might mistakenly count the router subinterfaces as separate broadcast domains or think that the router itself adds an extra broadcast domain, leading them to choose 4 instead of 3.
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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Related to this question
Learn chapter
Network Virtualization and VXLAN
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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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.