TCP Source Port Number Size in Bits
Short answer
A The Source Port Number in a TCP Header packet is 16 bits long.
Question
Key Takeaways
This question tests knowledge of TCP header structure fields and their bit lengths, specifically identifying the fixed 16-bit size of the source port identifier.
The TCP header's source port number field is exactly 16 bits long, a fundamental constant of the TCP/IP protocol suite defined in RFC 793. This page confirms that option A is the correct answer based on standard network architecture.
Common mistake: Learners sometimes confuse the 16-bit port number with the 32-bit sequence numbers or IP addresses, but the port field is strictly limited to 16 bits to support 65,536 ports per endpoint.
Expert Analysis
Why the Answer Is Correct
The Transmission Control Protocol (TCP) header contains a 16-bit source port field. This field is used by the source host to identify the specific application or process initiating the connection. Because it is 16 bits wide, it allows for values ranging from 0 to 65,535, which defines the range of available ports.
Why the Other Options Are Wrong
Option B (32) refers to fields like the Sequence Number or Acknowledgment Number, not the port. Option C (48) typically corresponds to MAC addresses (Layer 2), while Option D (64) does not correspond to any standard single-field size in the basic TCP header.
Community Comment Notes
The community consensus strongly supports option A, citing RFC 793 as the authoritative source. Comments clarify that the 16-bit size is a fixed part of the protocol specification, distinguishing it from other header fields that may be 32 bits.
Official Reference
https://www.rfc-editor.org/rfc/rfc793.txt
This link opens the official vendor documentation in a new tab.
Exam Strategy
Memorize the key bit-lengths of the TCP header: Source/Dest Ports are 16 bits each, Sequence/Ack Numbers are 32 bits each. Understanding these fixed sizes helps you quickly eliminate distractors in network forensics questions.
Frequently Asked Questions
Why is the source port 16 bits instead of 32?
Does the destination port also use 16 bits?
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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aqeel1506 👍 1The correct answer is A. 16. The Source Port Number in a TCP Header packet is a 16-bit field, which means it is 16 bits long. This field identifies the port number on the source device that is used to initiate the TCP connection. Here's a brief overview of the other options: B. 32 bits is the length of the Sequence Number and Acknowledgment Number fields in the TCP Header. C. 48 bits is not a standard field length in the TCP Header. D. 64 bits is the length of the Timestamps option in the TCP Header, which is optional and not related to the Source Port Number. The TCP Header format is defined in RFC 793, and the Source Port Number is specified as a 16-bit field in Section 3.1 of that document.
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044f354 👍 1 Answered AFrom the official specification for TCP, RFC 793 (IETF). https://www.ietf.org/archive/id/draft-ietf-tcpm-rfc793bis-25.html Answer: A. 16 Explanation: (A) ✅ 16 - The Source Port Number field in a TCP header is 16 bits long. (B) ❌ 32 - No field in the TCP header is 32 bits for port numbers. (C) ❌ 48 - Port numbers do not require 48-bit representation. (D) ❌ 64 - TCP uses 16-bit port numbers, not 64-bit values. ----- If you agree, please UPVOTE. Each time you upvote someone with a yellow "SELECTED ANSWER: " banner (like me) your vote is added to the Community Vote Distribution, which helps the community by crowdsourcing correct answers.
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Elb 👍 1A > Source TCP port number (2 bytes or 16 bits)
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