- A
Layer 7 – Application
Why wrong: Layer 7 involves application protocols like HTTP.
- B
Layer 3 – Network
Why wrong: Layer 3 deals with routing and IP addressing.
- C
Layer 4 – Transport
Port scanning uses TCP or UDP at Layer 4 to probe ports.
- D
Layer 2 – Data Link
Why wrong: Layer 2 handles MAC addressing and switching.
Question 779 of 1,000
CISSP Communication and Network Security Practice Question
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.
During a security assessment, a penetration tester sends TCP SYN packets to various ports on a target server. Based on the responses, the tester determines which ports are open. This technique is commonly used at which OSI layer?
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
✓
Layer 4 – Transport
The TCP SYN scan operates at Layer 4 (Transport) of the OSI model because it manipulates TCP segment headers, specifically the SYN flag, to probe port states. The tester sends SYN packets and interprets the response (SYN-ACK for open, RST for closed) to infer port availability, which is a transport-layer function defined by RFC 793.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Layer 7 – Application
Why it's wrong here
Layer 7 involves application protocols like HTTP.
- ✗
Layer 3 – Network
Why it's wrong here
Layer 3 deals with routing and IP addressing.
- ✓
Layer 4 – Transport
- ✗
Layer 2 – Data Link
Why it's wrong here
Layer 2 handles MAC addressing and switching.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the OSI layer of the scanning technique with the layer of the protocol being scanned (e.g., thinking a web server scan is Layer 7), but the SYN scan itself operates at Layer 4 regardless of the application running on the target port.
Detailed technical explanation
How to think about this question
During a SYN scan, the tester sends a TCP segment with the SYN flag set and the destination port number; if the target responds with SYN-ACK, the port is open, and the tester sends a RST to tear down the half-open connection, avoiding a full TCP handshake. This technique is often called 'stealth scanning' because it does not complete the three-way handshake, making it less likely to be logged by some applications. In real-world scenarios, firewalls may drop SYN packets to closed ports, causing the scan to show filtered ports instead of closed, which requires additional techniques like FIN or Xmas scans.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A security team runs a vulnerability scan on a web application and discovers an unpatched SQL injection flaw. The team prioritises remediation by CVSS score — critical flaws are patched within 24 hours, high within 7 days. Questions like this test whether you understand vulnerability management processes, scanning tools, and remediation prioritisation.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this CISSP question test?
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Layer 4 – Transport — The TCP SYN scan operates at Layer 4 (Transport) of the OSI model because it manipulates TCP segment headers, specifically the SYN flag, to probe port states. The tester sends SYN packets and interprets the response (SYN-ACK for open, RST for closed) to infer port availability, which is a transport-layer function defined by RFC 793.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 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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