CISSP Communication and Network Security Practice Question
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?
⚠ Common exam trap
The trap is that candidates may mistakenly associate the scanning technique with the application layer (Layer 7) because many protocols (HTTP, FTP) run on top of TCP, but the SYN scan itself operates at the transport layer (Layer 4) by manipulating TCP segment headers.
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Layer 7 – Application
Why it's wrong here
Port scanning via SYN packets operates at Layer 4, where TCP headers and port numbers are processed; Layer 7 concerns application protocols such as HTTP. It is tempting because open ports ultimately serve application services, and application-layer firewalls inspect payloads, but the SYN/ACK response itself is a transport-layer artefact.
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Layer 3 – Network
Why it's wrong here
Layer 3 routes packets using IP addresses and does not expose TCP port numbers or SYN flags, which belong to Layer 4 (Transport). It is tempting because SYN packets are IP packets, and Layer 3 would be correct for analysing ICMP sweeps, IP fragmentation or routing-based reconnaissance.
- ✓
Layer 4 – Transport
Why this is correct
TCP SYN scanning operates at Layer 4, the Transport layer, because it manipulates TCP flags and port numbers to infer open, closed or filtered states. Ports and TCP handshake behaviour are Transport-layer constructs, so interpreting SYN responses belongs to this layer.
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Layer 2 – Data Link
Why it's wrong here
TCP SYN scanning reads TCP header flags and port numbers, which are Layer 4 (Transport) constructs; Layer 2 addresses frames via MAC addresses and does not interpret ports. It is tempting because frames carry the encapsulated packets, and Layer 2 would be correct for analysing MAC flooding, ARP spoofing or switch CAM-table attacks.
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Security Assessment and Testing
Key term
Security
Security in IT is the practice of protecting systems, networks, and data from unauthorized access, damage, or theft.
Key term
Security assessment
A security assessment is a systematic evaluation of an organization’s systems, networks, and applications to identify vulnerabilities, threats, and risks, and to recommend improvements.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.