Which of the following is a 16-bit field that identifies the source port number of the application program in the host that is sending the segment?

Question

Options

• ASequence Number
• BHeader Length
• CAcknowledgment Number
• DSource Port Address

• A
  2% (1)
• B
  4% (2)
• C
  2% (1)
• D
  92% (49)

Explanation

Source Port Address (D) is a 16-bit field in the TCP/UDP segment header that identifies which application (port) on the sending host originated the data - this is its exact, defined purpose in transport-layer protocols.

Why the others are wrong:

• A. Sequence Number - a 32-bit field that tracks the byte order of data for reassembly and reliability, not the sender's application identity.
• B. Header Length - indicates how many 32-bit words make up the header itself, used to find where data begins.
• C. Acknowledgment Number - a 32-bit field used by the receiver to confirm which bytes it has successfully received, part of TCP's reliability mechanism.

Memory tip: Think of "Source Port Address" like a return address on an envelope - it tells the recipient which door (port/application) on the sender's machine to reply to. "Source = sender, Port = application door."

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