Quick Answer
The answer is ACK. This final segment is essential because the TCP three-way handshake steps require the client to acknowledge the server’s SYN-ACK, confirming that both sides have synchronized their sequence numbers and agreed on connection parameters. Without this ACK, the server would remain in a half-open state, unable to begin data transmission, as the handshake is only complete once the client sends this acknowledgment. On the CompTIA Network+ N10-009 exam, this question tests your understanding of the handshake’s state transitions and the necessity of the final step; a common trap is assuming the server’s SYN-ACK finishes the process, but the client must always reply with an ACK to establish a full-duplex connection. A helpful memory tip is to think of it as a polite conversation: SYN says “Hello,” SYN-ACK says “Hello back,” and ACK says “Got it, let’s talk.”
This N10-009 practice question tests your understanding of networking concepts. 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.
A client and server are establishing a TCP connection. The client sends a SYN segment to the server. The server responds with a SYN-ACK segment. What is the next segment in the handshake?
ACK
The final acknowledgment (ACK) confirms the server's SYN-ACK and completes the handshake.
RST
Why wrong: RST is used to reset a connection, not to complete the handshake.
FIN
Why wrong: FIN is used to gracefully close a connection, not to establish it.
SYN
Why wrong: A second SYN would indicate a new attempt, not a continuation of the handshake.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ACK
The TCP three-way handshake requires the client to acknowledge the server's SYN-ACK by sending an ACK segment. This completes the handshake, establishing a full-duplex connection with synchronized sequence numbers. Without this final ACK, the server remains in a half-open state, unable to begin data transmission.
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.
ACK
Why this is correct
The final acknowledgment (ACK) confirms the server's SYN-ACK and completes the handshake.
Related concept
Read the scenario before looking for a memorised answer.
RST
Why it's wrong here
RST is used to reset a connection, not to complete the handshake.
FIN
Why it's wrong here
FIN is used to gracefully close a connection, not to establish it.
SYN
Why it's wrong here
A second SYN would indicate a new attempt, not a continuation of the handshake.
Common exam traps
Cisco often tests the misconception that the handshake ends after the SYN-ACK, or that a FIN or RST could be used to complete the handshake, when in fact the final ACK is mandatory to transition the server's state from SYN-RECEIVED to ESTABLISHED.
Detailed technical explanation
During the TCP three-way handshake, the client's initial SYN carries an Initial Sequence Number (ISN), and the server's SYN-ACK acknowledges that ISN (by sending ACK = client_ISN + 1) while providing its own ISN. The final ACK from the client acknowledges the server's ISN (ACK = server_ISN + 1), and this segment may already contain payload data (as per RFC 1122). In real-world scenarios, this final ACK can be piggybacked with the first application data, reducing latency in protocols like HTTP.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Networking Concepts — This question tests Networking Concepts — Read the scenario before looking for a memorised answer..
The correct answer is: ACK — The TCP three-way handshake requires the client to acknowledge the server's SYN-ACK by sending an ACK segment. This completes the handshake, establishing a full-duplex connection with synchronized sequence numbers. Without this final ACK, the server remains in a half-open state, unable to begin data transmission.
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 11, 2026
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.
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