Quick Answer
The answer is the Sequence Number field in the TCP header, which is used to reassemble fragmented packets. This field works by numbering each byte of data in the TCP stream, allowing the receiver to reorder any IP-layer fragments back into the correct byte order based on the original sequence. When a TCP segment is fragmented at the IP layer, the sequence number acts as a map, ensuring that even if packets arrive out of order, the receiver can reconstruct the exact data stream. On the Cisco CyberOps Associate 200-201 exam, this concept tests your understanding of TCP reliability mechanisms versus IP fragmentation, often appearing in traffic analysis scenarios where an analyst must identify which header field enables proper reassembly. A common trap is confusing the TCP sequence number with the IP Identification field, but remember: IP IDs track datagram fragments, while TCP sequence numbers track the byte stream. Memory tip: think of the sequence number as a page number in a book—without it, you cannot reassemble the story in the right order.
This 200-201 practice question tests your understanding of network intrusion analysis. 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 an incident, an analyst captures network traffic. Which field in a TCP header is used to reassemble fragmented packets?
Flags
Why wrong: Incorrect. Flags control connection state, not reassembly.
Window size
Why wrong: Incorrect. Window size is used for flow control.
Acknowledgment number
Why wrong: Incorrect. ACK number is used for reliable delivery, not reassembly.
Sequence number
Correct. The sequence number indicates the order of bytes for reassembly.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Sequence number
The Sequence Number field in the TCP header is used to reassemble fragmented packets because it indicates the order of data bytes in the stream. When a TCP segment is fragmented at the IP layer, the sequence number allows the receiver to correctly reorder the fragments based on the original byte stream, ensuring proper reassembly.
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.
Flags
Why it's wrong here
Incorrect. Flags control connection state, not reassembly.
Window size
Why it's wrong here
Incorrect. Window size is used for flow control.
Acknowledgment number
Why it's wrong here
Incorrect. ACK number is used for reliable delivery, not reassembly.
Sequence number
Why this is correct
Correct. The sequence number indicates the order of bytes for reassembly.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the distinction between IP fragmentation (handled by the Identification field and fragment offset in the IP header) and TCP reassembly (handled by the Sequence Number), causing candidates to confuse the two layers.
Detailed technical explanation
In TCP, the Sequence Number is a 32-bit field that tracks every byte of data sent, starting from an initial sequence number (ISN) chosen during the three-way handshake. When IP fragmentation occurs, each fragment carries the same TCP header, and the receiver uses the sequence numbers to reorder the fragments before passing the complete segment to the application layer. A real-world scenario involves large TCP segments (e.g., 1500+ bytes) that exceed the MTU, causing IP fragmentation; without correct sequence numbers, reassembly would fail, leading to retransmissions.
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 200-201 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
Network Intrusion Analysis — This question tests Network Intrusion Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: Sequence number — The Sequence Number field in the TCP header is used to reassemble fragmented packets because it indicates the order of data bytes in the stream. When a TCP segment is fragmented at the IP layer, the sequence number allows the receiver to correctly reorder the fragments based on the original byte stream, ensuring proper reassembly.
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 25, 2026
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