This CISSP practice question tests your understanding of communication and network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which three are network-layer security controls in a defense-in-depth strategy? (Choose THREE.)
Antivirus
Why wrong: Host-based security control.
Access control lists (ACLs)
Filters traffic based on IP addresses and ports.
Data encryption at rest
Why wrong: Protects data at the storage layer.
Firewall
Controls traffic based on rules at the network layer.
Intrusion Detection System (IDS)
Detects malicious activity on the network.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Access control lists (ACLs)
Access control lists (ACLs) operate at Layer 3 by filtering traffic based on source and destination IP addresses, using rules that permit or deny packets as they traverse a router or Layer 3 switch. This makes ACLs a fundamental network-layer security control in a defense-in-depth strategy, as they enforce traffic segmentation and restrict unauthorized network access.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Antivirus
Why it's wrong here
Host-based security control.
Access control lists (ACLs)
Why this is correct
Filters traffic based on IP addresses and ports.
Related concept
Read the scenario before looking for a memorised answer.
Data encryption at rest
Why it's wrong here
Protects data at the storage layer.
Firewall
Why this is correct
Controls traffic based on rules at the network layer.
Related concept
Read the scenario before looking for a memorised answer.
Intrusion Detection System (IDS)
Why this is correct
Detects malicious activity on the network.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates confuse network-layer controls with host-based or data-at-rest protections, mistakenly selecting antivirus or encryption at rest because they are common security measures, but they operate at different layers of the OSI model.
Detailed technical explanation
Network-layer security controls operate on IP packets, inspecting headers and making decisions based on Layer 3 information such as source/destination IP, protocol type (e.g., TCP, UDP, ICMP), and sometimes port numbers via extended ACLs. In a defense-in-depth model, ACLs are often deployed on perimeter routers and internal firewalls to create security zones, while firewalls (stateful or stateless) and network-based IDS/IPS systems analyze traffic patterns and enforce policies at the same layer, complementing each other to block malicious traffic before it reaches higher layers.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
What to study next
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FAQ
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
The correct answer is: Access control lists (ACLs) — Access control lists (ACLs) operate at Layer 3 by filtering traffic based on source and destination IP addresses, using rules that permit or deny packets as they traverse a router or Layer 3 switch. This makes ACLs a fundamental network-layer security control in a defense-in-depth strategy, as they enforce traffic segmentation and restrict unauthorized network access.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CISSP practice question is part of Courseiva's free ISC2 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 CISSP exam.
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