N10-009 Subnet Mask Practice Question
Which of the following best describes the primary function of a subnet mask in IPv4 networking?
⚠ Common exam trap
CompTIA often tests the misconception that the subnet mask assigns IP addresses or acts as a gateway, so candidates confuse it with DHCP or the default gateway, especially when questions pair subnet mask with 'dynamic' or 'router' in the answer choices.
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
✓
It identifies which part of an IP address is the network address and which part is the host address
The subnet mask is a 32-bit value that separates the IP address into network and host portions through a bitwise AND operation. This allows routers and hosts to determine whether a destination IP is on the same local network or requires forwarding to a default gateway. Without the subnet mask, a device cannot distinguish the network prefix from the host identifier, making routing impossible.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
It identifies which part of an IP address is the network address and which part is the host address
Why this is correct
The subnet mask is used to divide the IP address into network and host bits, allowing devices to determine if a destination is on the same local network or requires routing.
- ✗
It dynamically assigns IP addresses to devices on the network
Why it's wrong here
IP address assignment is performed by DHCP, not by the subnet mask.
When this WOULD be correct
In a question asking 'Which protocol dynamically assigns IP addresses to devices on a network?', the correct answer would be DHCP, which is often described as dynamically assigning IP addresses.
- ✗
It encrypts data packets to ensure secure communication
Why it's wrong here
Encryption is provided by protocols such as IPsec or TLS, not by a subnet mask.
When this WOULD be correct
In a question asking 'Which technology provides confidentiality for data transmitted over an IP network?', 'IPsec' or 'encryption' would be correct, but not a subnet mask. If the option were 'It encrypts data packets to ensure secure communication' and the question were about the function of IPsec, then it would be correct.
- ✗
It specifies the IP address of the router that forwards traffic to other networks
Why it's wrong here
The default gateway is configured manually or via DHCP, and is a separate parameter from the subnet mask.
When this WOULD be correct
In a question asking 'Which of the following best describes the function of a default gateway in IPv4 networking?', option D would be correct because the default gateway specifies the router IP for forwarding traffic to other networks.
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.
✓It identifies which part of an IP address is the network address and which part is the host addressCorrect answer▾
Why this is correct
The subnet mask is used to divide the IP address into network and host bits, allowing devices to determine if a destination is on the same local network or requires routing.
✗It dynamically assigns IP addresses to devices on the networkWrong answer — click to see why▾
Why this is wrong here
A subnet mask does not assign IP addresses; it is used to determine the network and host portions of an IP address. Dynamic IP assignment is performed by DHCP, not the subnet mask.
★ When this WOULD be the correct answer
In a question asking 'Which protocol dynamically assigns IP addresses to devices on a network?', the correct answer would be DHCP, which is often described as dynamically assigning IP addresses.
Why candidates choose this
Candidates may confuse the subnet mask with network configuration tasks, mistakenly thinking it handles IP address assignment because both are involved in network setup.
✗It encrypts data packets to ensure secure communicationWrong answer — click to see why▾
Why this is wrong here
A subnet mask does not encrypt data packets; encryption is handled by protocols like IPsec or TLS, not by the subnet mask.
★ When this WOULD be the correct answer
In a question asking 'Which technology provides confidentiality for data transmitted over an IP network?', 'IPsec' or 'encryption' would be correct, but not a subnet mask. If the option were 'It encrypts data packets to ensure secure communication' and the question were about the function of IPsec, then it would be correct.
Why candidates choose this
Candidates may confuse the subnet mask's role in network segmentation with security functions, or they may think that subnetting inherently provides security through isolation.
✗It specifies the IP address of the router that forwards traffic to other networksWrong answer — click to see why▾
Why this is wrong here
The subnet mask does not specify the default gateway; it only delineates network and host portions of an IP address. The default gateway is a separate configuration parameter.
★ When this WOULD be the correct answer
In a question asking 'Which of the following best describes the function of a default gateway in IPv4 networking?', option D would be correct because the default gateway specifies the router IP for forwarding traffic to other networks.
Why candidates choose this
Candidates often confuse the subnet mask with other network configuration parameters like the default gateway, especially when studying basic networking concepts together.
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
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
Go deeper
Related to this question
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Cloud Networking Concepts
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.
Key term
Default gateway
A default gateway is a network device, typically a router, that acts as the exit point for traffic from a local network to other networks, including the internet.
About these practice questions
Courseiva writes every N10-009 question from scratch — 472 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
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.