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TCP Header Analyzer

Decode a TCP header from hex — ports, flags, window and sequence.

  • Free
  • No signup
  • Nothing uploaded
  • No watermark

Source port

80

Destination port

443

Window

8192

Header length

20 bytes

Flags set

SYNopening a connection

Sequence number
1
Acknowledgement
0
Checksum
0x0000
Urgent pointer
0

This is the first packet of a handshake — a client opening a connection.

How to Use the TCP Header Analyzer Online

TCP Header Analyzer: before and after TEXT A YOUR INPUT TCP HEADERANALYZER DONE A RESULT

Decodes a TCP header from hex into ports, flags, sequence numbers and window size. Reading the flags is usually the point: a connection that never completes shows it in the SYN and ACK pattern, and a window of zero explains a transfer that has stalled without failing.

No software to install Free Works on any device

Step-by-Step Guide to Using the TCP Header Analyzer

  1. 1. Paste the TCP header as hex — at least 20 bytes

  2. 2. Every problem is listed as you type, with what it means and why it matters

  3. 3. Nothing is fetched, so this works on staging sites and pages behind a login

Try it now

What is the TCP Header Analyzer?

Decodes a TCP header from hex into ports, flags, sequence numbers and window size. Reading the flags is usually the point: a connection that never completes shows it in the SYN and ACK pattern, and a window of zero explains a transfer that has stalled without failing.

Why Use IP and Network Calculators?

Subnet masks, CIDR ranges and download times all come down to binary maths that is easy to slip on. These calculators do the sums in the page, so you can plan an address range or check a speed claim with confidence.

“One wrong subnet mask can take a whole office offline.”
  • Subnets worked out instantly
  • CIDR ranges made clear
  • Download times estimated
  • Browser details revealed
  • Usable host counts shown
  • IPv4 and IPv6 conversions
  • MAC addresses checked
  • No commands to install

Popular Uses for Subnet Calculators

Network jobs at home and at work often start with one of these sums.

  • Planning an office network Split an address block into subnets for each floor, team or guest WiFi.
  • Reading CIDR notation See the first address, last address and host count behind a range like /24.
  • Download estimates Work out how long a large file will take on a given connection speed.
  • Speed claims Turn megabits per second into megabytes to see what a plan really delivers.
  • Device lookups Find the maker of a network card from its MAC address prefix.
  • Browser checks See your user agent and browser details when a support team asks.
  • WebSocket tests Connect to a WebSocket endpoint and watch messages go back and forth.
  • IPv6 planning Convert an IPv4 address into IPv6 form while preparing a migration.

What You Can Do with the TCP Header Analyzer

Shows the network, broadcast and usable host range together, which is the whole answer

Handles IPv6 at full length rather than truncating it for display

Why Use ToolXkit for TCP Header Analyzer?

Your files stay with you

Everything runs inside your browser. Nothing you open or type is uploaded to a server.

Network Tools for Different Users

Network administrators

Plan subnets, check CIDR ranges and size links for expected bandwidth.

Networking students

Practise subnetting for CCNA and other exams, and check answers quickly.

Developers

Test WebSocket servers, parse user agents and inspect TCP header fields.

IT support staff

Collect browser details and estimate transfer times while helping users.

Small office owners

Understand router settings and IP ranges without calling a technician every time.

Streamers and gamers

Check upload speed, latency and packet loss figures before a stream.

Best Times to Work Out a Subnet

Network calculators help when you are:

  • Setting up a new router
  • Studying for a networking exam
  • Adding a guest WiFi network
  • Estimating a large upload
  • Comparing broadband plans
  • Identifying an unknown device
  • Moving to IPv6
  • Debugging a WebSocket connection

Frequently Asked Questions

Answers to common questions about this tool.

What do the TCP flags tell me?

They show where a connection is in its lifecycle. SYN opens, SYN-ACK accepts, ACK confirms, FIN closes politely and RST closes abruptly. A capture full of SYNs with no SYN-ACK means nothing is listening or a firewall is silently dropping; RST means something actively refused.

What does a zero window mean?

The receiver has told the sender to stop — its buffer is full because the application is not reading fast enough. It is a stalled transfer that has not failed, and the cause is at the receiving application rather than anywhere in the network.

Is TCP Header Analyzer accurate enough for real network work?

The maths behind addresses and ranges is exact, so the results match what a router expects. Double-check the input, especially the prefix length, since one wrong digit changes the whole range.

Do the calculators scan my network?

No. The subnet and speed calculators only work with the numbers you type in. They send no traffic into your network, and the only tool that connects anywhere is the WebSocket Tester, to the address you enter.

Why is my real download slower than the estimate?

The calculator assumes the full connection speed is available. In practice, WiFi, other devices, server limits and protocol overhead eat into it, so real downloads often take somewhat longer.

Is TCP Header Analyzer free and private?

Yes. It costs nothing and needs no account. What you type stays in your browser and is never sent to ToolXkit, which matters when it describes a real company network.

What is the difference between Mbps and MBps?

Mbps means megabits per second, the unit broadband plans use. MBps means megabytes per second, the unit most download windows show. One byte is eight bits, so 100 Mbps is about 12.5 MBps.

Further reading

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