TCP Sequence Number Utility

TCP Sequence Number Calculator

Calculate the next TCP sequence number, expected acknowledgment number, sequence-space consumption, payload byte range and 32-bit wraparound from SEQ, payload length, SYN and FIN flags.

✓ Next SEQ ✓ ACK Number ✓ SYN / FIN ✓ Payload Range ✓ 32-bit Wraparound ✓ Hex + Decimal

SEQ

TCP Sequence Calculation

● Ready

Starting TCP Sequence Number Enter decimal or hexadecimal with a 0x prefix. Valid TCP range is 0 through 4,294,967,295.

TCP Payload Length Application-data bytes carried by this TCP segment.

SYN Flag A SYN consumes one position in TCP sequence space.

FIN Flag A FIN also consumes one position in TCP sequence space.

Current ACK Number Optional existing ACK field for display. It does not change the sender’s next sequence number.

Number Display

TCP rule: the next sequence number is SEQ + payload bytes + SYN + FIN, calculated modulo 2^32. The ACK flag itself consumes no sequence number.

TCP Sequence Calculation Result Calculated

Next TCP Sequence Number —

Starting SEQ —

Payload Bytes —

SYN Consumption —

FIN Consumption —

Total Consumed —

Next SEQ —

Expected ACK —

Current ACK —

Payload First SEQ —

Payload Last SEQ —

Wraparound —

Sequence Space —

Sequence-Space Layout —

Calculation Breakdown —

Validation —

What Is a TCP Sequence Number?

TCP assigns sequence numbers to bytes in its reliable byte stream. The Sequence Number field in the TCP header identifies the sequence-space position associated with the segment.

Sequence numbers are 32 bits wide, so they range from 0 through 4,294,967,295 before wrapping back to zero.

TCP Next Sequence Number Formula

Sequence Space Consumed =

Payload Length
+
SYN
+
FIN

Next SEQ =

(Starting SEQ + Sequence Space Consumed)
mod 2^32

SYN and FIN are counted as one sequence-space position each. Other ordinary TCP flags such as ACK, PSH and RST do not add payload sequence bytes by themselves.

TCP Payload Sequence Number Example

Suppose a segment starts with sequence number 1000 and carries 500 bytes with no SYN or FIN.

Starting SEQ:
1000

Payload:
500 bytes

Payload sequence numbers:
1000 through 1499

Next SEQ:
1500

Receiver ACK:
1500

Why Does SYN Consume One Sequence Number?

Although SYN is a control flag rather than application payload, TCP assigns it one position in sequence space. This is why the first data byte following an initial SYN normally starts at ISN + 1.

SYN Segment:

SEQ = 1000
SYN = 1
Payload = 0

Next SEQ:
1001

Expected ACK:
1001

Why Does FIN Consume One Sequence Number?

FIN also occupies one sequence-space position. A receiver acknowledges the FIN by advancing its acknowledgment number by one after all preceding payload bytes.

SEQ:
5000

Payload:
100 bytes

FIN:
1

Sequence consumption:
100 + 1 = 101

Expected ACK:
5101

TCP SYN with Payload

If SYN and payload occur in the same segment, the SYN consumes the first sequence-space position and payload begins immediately after it.

SEQ = 1000
SYN = 1
Payload = 10 bytes

SYN occupies:
1000

Payload occupies:
1001 through 1010

Next SEQ:
1011

TCP Sequence Number Wraparound

TCP sequence arithmetic operates in a 32-bit space. When addition exceeds 4,294,967,295, the result wraps modulo 2^32.

Starting SEQ:
4294967290

Payload:
10

Calculation:
4294967290 + 10
= 4294967300

2^32:
4294967296

Wrapped Next SEQ:
4

TCP Acknowledgment Number

A TCP acknowledgment number normally identifies the next sequence number the receiver expects from the other endpoint.

Received segment:

SEQ = 2000
Payload = 100 bytes

Bytes received:
2000–2099

Expected ACK:
2100

If the segment contains SYN or FIN, their sequence-space consumption must also be included when calculating the acknowledgment.

TCP Sequence Number vs ACK Number

Field	Meaning
Sequence Number	Position of the segment in the sender’s TCP sequence space
Acknowledgment Number	Next sequence-space position expected from the peer
Payload Length	Number of application-data bytes consuming sequence space
SYN	Consumes one sequence number
FIN	Consumes one sequence number
ACK Flag	Does not itself consume sequence space

TCP Initial Sequence Number

When a TCP connection begins, each endpoint selects an Initial Sequence Number, or ISN. During the three-way handshake the SYN consumes one position, so the next sequence number used by that endpoint is ISN + 1 modulo 2^32.

TCP Three-Way Handshake Example

Client → Server

SYN
SEQ = 1000

Server expects:
ACK = 1001


Server → Client

SYN, ACK
SEQ = 5000
ACK = 1001

Client expects:
ACK = 5001


Client → Server

ACK
SEQ = 1001
ACK = 5001

TCP Sequence Number Calculator FAQs

How do I calculate the next TCP sequence number?

Add the TCP payload length plus one for SYN if present and plus one for FIN if present, then apply modulo 2^32 arithmetic.

Does the TCP ACK flag consume a sequence number?

No. The ACK flag itself does not consume sequence space.

Does SYN consume a sequence number?

Yes. SYN consumes one TCP sequence-space position.

Does FIN consume a sequence number?

Yes. FIN consumes one sequence-space position.

What is the maximum TCP sequence number?

A TCP Sequence Number field is 32 bits, so its maximum unsigned value is 4,294,967,295, or 0xFFFFFFFF.

What happens after TCP sequence number 0xFFFFFFFF?

Sequence arithmetic wraps modulo 2^32, so the value following 0xFFFFFFFF is 0x00000000.

How do I calculate the expected TCP ACK number?

For an in-order segment, add the number of sequence-space positions consumed by its payload, SYN and FIN to its starting sequence number.

Does PSH consume a TCP sequence number?

No. PSH does not independently consume sequence space. Any payload carried in the segment does consume sequence numbers.