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Acknowledgment number

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

An acknowledgment number is a TCP value identifying the next sequence number or byte the receiver expects.

An acknowledgment number is the TCP sequence number of the next segment the receiver expects. It confirms receipt and supports reliable, ordered communication. section 22.2.1

TCP hosts maintain separate sequence and acknowledgment numbers in each direction. After receiving a segment, a host acknowledges it by reporting the next expected sequence number—not the number just received. section 22.2.1

Reading the next expected number

If PC1 sends a segment with sequence number 10, SRV1 replies with acknowledgment number 11. If SRV1 sends sequence number 50, PC1 acknowledges it with 51. section 22.2.1

Important TCP behaviors include:

  • A single acknowledgment can acknowledge multiple segments when data arrives in sequence. section 22.2.1
  • If an acknowledgment does not arrive before the retransmission timeout, the sender retransmits the segment. section 22.3
  • Acknowledgment numbers work with sequence numbers and the Window Size field to support TCP sequencing, reliability, and flow control. section 22.3

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Explains next-sequence-number acknowledgment, bidirectional tracking, cumulative acknowledgments, and retransmission, without packet-capture or header-analysis procedures.
Examples
Uses PC1 and SRV1 exchanges with sequence-to-acknowledgment transitions from 10 to 11 and from 50 to 51.
What it assumes
Assumes basic TCP segment awareness while establishing sequence numbers, acknowledgments, and reliable delivery through concrete exchanges.
What you can do after
Diagnose TCP acknowledgment behavior by identifying the next expected sequence number and recognizing retransmission after timeout.
How it connects
Connects acknowledgment numbers to TCP, sequence numbers, reliable communication, retransmission, and sliding-window flow control.
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