Which TCP Header Field Does This?

About this interactive

From the lesson, field by field (the ports, 16 bits each, open the header): - **Sequence number (32 bits):** the number of the first data byte in this segment. The first SYN carries a random starting value. - **Acknowledgment number (32 bits):** the next byte the receiver expects. It means nothing until the ACK flag is on. - **Data offset (4 bits):** the header's length in 4-byte rows: 5 with no options (20 bytes), more when options are added, padding filling the last row. It tells the receiver where the data starts. Wireshark calls it the header length. - **Flags:** single bits, each on or off. SYN asks to synchronize sequence numbers, ACK says the acknowledgment number is in use, FIN says this host has finished sending. - **Window size:** how much more data the receiver will accept right now. - **Checksum:** an error check over the header, the data and the IP addresses (from the IP header). - **Urgent pointer:** marks urgent data when the URG flag is on.

What you'll learn

Identify the TCP header field behind a job: sequence number, acknowledgment number, flags, data offset, window size or checksum
Tell the ACK flag from the acknowledgment number

Aligned to

CompTIA Network+

1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.

Cisco CCNA

1.5 Compare TCP to UDP

Key terms

Sequence Number: A 32-bit field in the TCP header that identifies the byte position of the first byte of data in a segment, allowing the receiver to reorder segments that arrive out of sequence and detect missing data. The receiving side uses the sequence number to send an acknowledgment confirming which byte it expects next.
Acknowledgment Number: A 32-bit field in the TCP header that indicates the next byte the receiver expects, confirming successful receipt of prior data.
TCP Flags: Single-bit fields in the TCP header — including SYN, ACK, and FIN — that control connection establishment, maintenance, and termination.
Window Size: A TCP header field that specifies how many bytes the receiver is willing to accept at one time, enabling flow control between devices.
Header: Control information prepended to the payload during encapsulation that supports delivery functions such as addressing, sequencing, and protocol identification.

Topics

Tcp Tcp Header Sequence Number Tcp Flags Window Size Checksum Interactive Categorize

About TechKnowSurge

TechKnowSurge builds IT and cybersecurity professionals through hands-on, concept-first training built around real understanding — not memorization. Free interactive tools, structured programs, and 25+ years of real-world experience, all in one place.

Explore free tools and programs →