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When Factorio multiplayer cannot connect, start with the three conditions that determine whether a session can work at all: every game instance must use exactly the same Factorio version and mods, the joining method must reach the host, and the server must accept Factorio’s UDP traffic. For an Internet-hosted game, that normally means the host’s public IP address, the configured UDP port (34197 by default), router forwarding to the host computer, and a firewall rule that permits the game.
Use the least fragile connection path first: a Steam invite, the public server browser, or Play on LAN when everyone is on one network. Move to direct IP:port joining only when necessary. If the host is behind NAT and cannot configure port forwarding, a virtual-LAN solution can provide a private network path. Dedicated servers require an additional check of the launch command, port, bind address, and client test command.
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The fastest fix depends on who fails to connect and where the boundary is. Do not change several settings at once; identify the first point at which the connection breaks.
- Nobody can join: verify that the server process is running, then check version and mod parity, the configured UDP port, router forwarding, and the host firewall.
- Players on the same network can join, but Internet players cannot: the game itself is working; investigate public reachability, NAT, port forwarding, or firewall policy. Try a Steam path or a virtual-LAN path to confirm the diagnosis.
- Only one person cannot join: compare that player’s Factorio version and mods, Steam status, local firewall or antivirus behavior, and the exact address and port entered.
- The server appears in a list but the connection fails: listing and reachability are separate. A server can be advertised while UDP acceptance or NAT traversal still fails.
- Only a dedicated server fails: launch it with a save, inspect its
--portand--bindsettings, and test from a client with--mp-connect ADDRESS.
Once you know which branch applies, use the corresponding section below instead of repeatedly retrying the same join button.
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Make every game instance identical
Factorio’s multiplayer requirement is exact: “All game instances need to be exactly the same game-versions and mods.” A small mismatch can prevent a player from joining even when the address, port, and router are correct.
Check the Factorio version
- On every computer, open Factorio and note the full version shown by the game client.
- Compare the host, dedicated server, and each player character by character. A stable release and an experimental release are different versions for multiplayer purposes.
- Update or roll back the outlying installation so that all instances use the same release before testing networking.
Do this comparison again after an update. If the host starts a new version while a friend’s client remains on the previous build, networking tests will produce a misleading “cannot connect” diagnosis.
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- Compare the enabled mod list on the host and every client.
- Match both the mod names and their versions. Having a mod installed but disabled is not the same state as having it enabled.
- For a dedicated server, make the server’s mod directory and enabled set agree with the clients before launching the save.
- Restart Factorio after changing the set, then attempt the join again.
For a quick isolation test, have the host create or load a save with no optional mods and ask the affected player to join. If the unmodded session works, re-enable the same mods and versions on every instance in a controlled batch until the mismatch is found.
Choose the right way to join
Factorio supports Steam invites, the public server browser, direct IP and port entry, and LAN play. Each method depends on different network conditions.
| Connection method | Best fit | What it requires |
|---|---|---|
| Steam invite | Friends already connected through Steam | Steam availability for the players and a running host session |
| Public server browser | A server intended to be discoverable | The server must be listed and still reachable over UDP |
| Play on LAN | Everyone is on the same local network | All computers can reach the host on the LAN |
| Direct IP:port | A known host address or a dedicated server | The correct address, configured UDP port, and public reachability when outside the LAN |
| Virtual LAN | The host cannot configure router forwarding | A virtual-LAN network shared by the host and players |
Start with the first method that fits your situation. Direct addressing is not automatically better; it exposes every port, firewall, and NAT assumption, so it is most useful when you control the server and its network.
Playing on the same local network
When all players are connected to the same home, office, or campus network, use Play on LAN. This avoids the Internet-facing NAT boundary and does not require the router to publish a port to the outside world.
- Confirm that the host and every player are on the same local network, not merely connected to the same Internet account.
- Start the Factorio session on the host.
- Have the other players use the LAN option and select the visible game.
- If the session is not shown, use the host’s local address with the configured port as a direct target.
- Check the host computer’s local firewall. It must permit Factorio’s UDP traffic from the local network.
A LAN success is a useful diagnostic result: it proves that the save, version, mods, and basic server process work. If players outside the network still fail, focus on the public address, NAT, forwarding, or firewall rather than reinstalling the game.
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Using Steam or the public server browser
Steam invite
A Steam invite is usually the least fragile Internet option for a small group because players do not have to type an address and port manually. The host starts the multiplayer game, sends the invite through Steam, and the friend accepts it while running the matching Factorio version and mods.
If the invite is unavailable or acceptance fails, check that both players are online in Steam and that each is running the same Factorio branch. A successful invite does not remove the version, mod, or firewall requirements; it only selects a different discovery and connection path.
Public server browser
Use the public browser when the server is intended to be discoverable. Search for the server, select it, and connect after confirming that your client’s version and mods match the server.
A listing is not proof that a connection can be completed. Factorio’s matching and discovery process can show a server even when the final UDP path is blocked or NAT traversal fails. If a listed server repeatedly rejects the connection, switch to a direct address supplied by the host or test a virtual-LAN route.
Joining by direct IP and port
Direct joining requires the exact address and port the server is listening on. For an Internet host, use the host’s public IP address, not the host computer’s private address such as an address assigned only inside the home network. Enter the address in Factorio’s direct-connect field in IP:port form.
Factorio’s transport and default port
Factorio uses UDP only. The default server port is UDP 34197, although the administrator can change it. A TCP-only firewall rule or a forwarding rule for the wrong protocol will not establish a Factorio session.
Forward the port to the host
- Find the port configured by the Factorio server. If no custom value is set, begin with UDP 34197.
- In the router’s port-forwarding section, create a rule for that UDP port.
- Set the destination to the host computer’s current private LAN address.
- Save or apply the rule, then confirm that the host computer still has that address. A changed private address can silently invalidate the forwarding rule.
- Allow the same UDP port through the host operating system’s firewall and any security product that filters network traffic.
- Give remote players the router’s current public IP address and the configured port.
Do not forward a port to a different computer, and do not provide players with the private address when they are outside the LAN. If the server uses a non-default port, every player must use that value in the direct-connect address.
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When NAT prevents a direct connection
Network address translation (NAT) allows multiple devices to share a public address, but it also means unsolicited Internet traffic normally cannot reach the host. A server behind NAT needs a publicly reachable address through port forwarding unless all players are on the same LAN.
If you cannot configure the router—because it is managed by an employer, school, landlord, mobile provider, or another upstream network—direct hosting may not be possible. In that case, use a virtual-LAN tool that places the host and players on the same private overlay network, then select the host’s virtual-LAN address and Factorio port. Every participant must join the same virtual network, and the host firewall must allow traffic from that virtual interface.
A virtual LAN changes the path; it does not fix a version or mod mismatch. Confirm identical game instances first, and keep the virtual network limited to people who should access the game.
Dedicated and headless server checks
A headless server runs without the normal game interface. The launch command must identify a save, and the process must listen on an address and port that players can reach.
Start the server with a save
Launch the dedicated process with:
factorio --start-server SAVEReplace SAVE with the save file or path used by your installation. Starting the process without a valid save does not provide a playable session for clients.
Verify the listening settings
Inspect the server configuration and command line for --port and --bind:
--portdetermines the UDP port. If it is not customized, the Factorio default is 34197.--binddetermines which local address the process listens on. A binding restricted to the wrong interface can make the server unreachable even when the process is running.
Make the router-forwarding rule, firewall rule, and player’s direct address agree with those settings. A custom port in the server configuration requires the same custom port in every network rule and join address.
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Test from a client
Factorio provides a client connection test using:
factorio --mp-connect ADDRESSUse the server address, including the port when the server is not using the default. This test helps separate a server-process problem from a discovery problem: if the explicit connection succeeds, the server is reachable and the browser or invite path is the part to investigate.
Firewall and antivirus interference
A firewall or antivirus product can block Factorio’s UDP traffic even when the router is configured correctly. Check both layers:
- Host operating-system firewall: permit the Factorio executable or the configured UDP port on the network profile used by the host.
- Router firewall: allow the forwarded UDP port and ensure the forwarding rule targets the correct private address.
- Security software: inspect its blocked-application or network-protection events and allow Factorio if it has quarantined or denied the connection.
- Client firewall: if only one player fails, check that player’s outbound and inbound filtering as applicable.
Temporarily disabling protection can be a diagnostic test, but restore it immediately and create a narrow Factorio rule instead of leaving the computer unprotected. If changing the rule makes no difference, re-enable the protection and continue with address, version, and NAT checks.
Why a listed server can still reject you
Factorio can use matching-server discovery, keepalives, and NAT punching to help players find one another. The development team reported an expected NAT-punching success rate of about 70% in its 2016 technical note. That figure is a historical expectation under the conditions tested at the time, not a current guarantee that every listed server will accept a connection.
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This explains the apparently contradictory state in which a server appears in a list but the join attempt times out. Discovery traffic reached the matching service, while the final UDP path between player and server did not complete. Port forwarding, a Steam route, or a virtual-LAN connection can bypass that limitation.
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Nobody can join a hosted game
- Confirm the host is actually running the save and has not closed the session.
- Match Factorio versions and enabled mods on every instance.
- Confirm the server’s UDP port, beginning with 34197 if no custom port is configured.
- Forward that UDP port to the host’s private address.
- Permit the port or Factorio executable through the host firewall and security software.
- Have one remote player test the public address directly.
If the direct test fails but LAN play works, the fault is at the public-network boundary. If LAN play also fails, return to the process, version, mod, and local firewall checks.
LAN works but Internet play fails
The game and save are functioning. Verify the public IP, UDP forwarding, firewall policy, and NAT restrictions. If forwarding cannot be changed, move the group to Steam or a virtual-LAN path rather than repeatedly retrying the private LAN address from outside.
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Only one friend cannot join
Compare that player’s full version and mod set with the host, then confirm the player is using the exact address and port. Check Steam connectivity and the player’s local firewall or antivirus. Because other players can connect, changing the router is unlikely to be the first useful step.
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Ask the host to verify the server process, configured port, bind address, and forwarding destination. Confirm that the address is public and current. A private LAN address will work only for players on the same network.
The server shows up but fails at the final connection
Treat the listing as a discovery result, not a reachability result. Test a Steam invite, direct address, or virtual-LAN path. If all paths fail, return to UDP acceptance and NAT rather than assuming the listing proves the server is healthy.
A repeatable troubleshooting order
Use this order to avoid changing unrelated settings:
- Identity: match the exact Factorio version and enabled mods.
- Process: verify that the host or headless server has started the intended save.
- Path: choose LAN, Steam, browser, direct IP, or virtual LAN based on where the players are located.
- Address: use a local address on LAN and a public address for Internet players.
- Transport: confirm UDP and the configured port; use 34197 only when the server has not changed it.
- Reachability: forward the port to the host and allow it through the relevant firewalls.
- Isolation: test one client with
--mp-connect ADDRESSfor a dedicated server or use one direct-connect attempt for a hosted game. - Fallback: if NAT cannot be configured, use Steam or a shared virtual-LAN network.
Record which step changes the result. That tells you whether the original failure was caused by game-state mismatch, a stopped process, an incorrect address, blocked UDP, or an unavoidable NAT boundary.
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What the multiplayer network model means for performance
In client-server mode, multiplayer traffic is relayed through the server. This allows players on a LAN and players outside it to participate in the same session, but a player may incur an additional network hop compared with a purely local connection. A successful connection therefore does not guarantee identical latency for every participant; the route to the server still matters.
Keep the server computer connected to a stable network, avoid changing its private address after forwarding the port, and keep the configured port consistent across the server, router, firewall, and join address. These operational details prevent a working setup from appearing broken after a restart or network change.
Quick Recap
Final preflight checklist
- Every player and the server run exactly the same Factorio version.
- The enabled mod names and versions match on all instances.
- The host has loaded the intended save and remains connected.
- The selected join method fits the network: LAN for one local network, Steam or browser for supported discovery, direct IP for controlled hosting, or virtual LAN when forwarding is unavailable.
- The address is local for LAN players and public for Internet players.
- Factorio’s UDP port is known; the default is 34197 unless changed.
- The router forwards that UDP port to the correct host computer.
- The host firewall and security software permit Factorio traffic.
- A dedicated server was started with
--start-server SAVEand has correct--portand--bindsettings. - An explicit client test with
--mp-connect ADDRESSconfirms reachability when discovery is unreliable.
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