200-901 Network Fundamentals Practice Question
A network engineer captures traffic between a client and a server. The client sends a TCP segment with the SYN flag set. The server responds with a segment that has both SYN and ACK flags set. Which field in the server's response acknowledges the client's initial sequence number?
⚠ Common exam trap
Test-takers frequently confuse the sequence number field, which carries the sender's own ISN, with the acknowledgment number field, which confirms the peer's sequence number during the handshake.
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
✓
The acknowledgment number field, set to the client's initial sequence number plus one.
During the TCP three-way handshake, the server's SYN-ACK segment acknowledges the client's SYN by placing the client's initial sequence number plus one in the acknowledgment number field. This confirms receipt of the SYN and establishes the client's sequence space. The sequence number field carries the server's own ISN, while window size and urgent pointer serve flow control and urgent data handling, respectively.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The window size field, set to the number of bytes the server can receive.
Why it's wrong here
The window size field advertises the amount of receive buffer space the server has available for incoming data. It is part of TCP flow control and does not acknowledge sequence numbers. In the handshake, it indicates how much data the client may send before waiting for an acknowledgment, but it does not confirm receipt of the client's SYN. Therefore, it is unrelated to acknowledging the initial sequence number.
- ✓
The acknowledgment number field, set to the client's initial sequence number plus one.
Why this is correct
In the TCP three-way handshake, the server's SYN-ACK segment acknowledges the client's SYN by setting the acknowledgment number to the client's initial sequence number (ISN) plus one. The plus-one accounts for the SYN flag consuming one sequence number. This field is what tells the client that the server received the SYN and expects the next byte from the client to be that value, completing the second step of the handshake.
- ✗
The urgent pointer field, set to indicate out-of-band data.
Why it's wrong here
The urgent pointer field is used with the URG flag to point to the end of urgent data in a TCP segment. It has no role in the three-way handshake and does not acknowledge sequence numbers. In a SYN-ACK segment, this field is typically zero and ignored. Using it to acknowledge the client's initial sequence number would be incorrect because acknowledgment is handled exclusively by the acknowledgment number field.
- ✗
The sequence number field, set to the server's own initial sequence number.
Why it's wrong here
The sequence number in the server's SYN-ACK represents the server's own initial sequence number for data it will send, not an acknowledgment of the client's SYN. It is used by the client to acknowledge the server's SYN in the third step. While it is present in the same segment, it does not perform the acknowledgment function described in the question, so it is not the field that acknowledges the client's initial sequence number.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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