Quick answer: Factorio connection issues: port 34197 UDP, visibility: public in server-settings.json and matching mods.
If you landed here: Factorio shows "Connection failed", mod mismatch or server not findable.
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Frequently asked questions (FAQ)
Which port for Factorio server?
34197 UDP (default). Open in server-settings.json and firewall/panel.
Factorio mod mismatch?
Mod versions on server and client must exactly match. Check server-mod-settings.json.
Factorio server not in LAN/internet list?
visibility public in server-settings.json. Port 34197 UDP open. Direct connect: IP:34197