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🚩 TCP Flags Reference
Eight TCP control flags fit in one byte. Different combinations mean opening a connection, acknowledging data, closing politely, or aborting hard. This reference walks through each one and the canonical sequences (three-way handshake, four-way close).
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Examples
Three-way handshake
Output
Client → SYN · Server → SYN+ACK · Client → ACK · Connection openFrequently asked questions
SYN+ACK vs SYN alone?
SYN starts a connection. SYN+ACK is the server saying "I got your SYN, here is mine." Different state transitions.
About this calculator
TCP Flags Reference runs entirely in your browser using standard formulas. No data is sent to any server. We don't share your inputs with lenders, brokers, or anyone else — there's no funnel and no follow-up email.
Related tools
- 🔌 TCP / UDP Port Reference — Searchable reference of common TCP/UDP ports and the services that use them.
- 🌐 HTTP Status Codes — Searchable reference of HTTP status codes (1xx, 2xx, 3xx, 4xx, 5xx).
Why this tool
vs typical free network-tools sites
| NetToolset | Typical free site | |
|---|---|---|
| Price | Free | Free |
| Signup required | No | Often (for advanced features) |
| Ads inside the tool | No | Yes (banner + interstitial) |
| Logs your IP / queries | No | Yes (often resold) |
| Data sent to a server | Math runs in browser; lookups hit Cloudflare DoH directly | Yes (all goes through their server) |
| Open source | Yes | No |
| Bookmarkable URL per tool | Yes | Mixed |
| Multilingual | EN / ES / PT / FR / DE | Usually EN only |
| Loads in under 1s | Yes (static) | Often slow (ad tracking) |