The easiest way to join a Core Keeper multiplayer world is with its Game ID: the host starts the world, opens the pause menu, copies the ID, and sends it to the other players. Each player selects Join Game on the main menu and enters that ID. The host must keep a normal hosted world open for the session.
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For a world that should stay available without a player actively hosting, install Core Keeper Dedicated Server from Steam’s Tools section. Launch it, copy the Game ID generated in the server window, and leave the server process running. Direct IP connections are an advanced alternative: launching with -port requires an open firewall path and, for internet play, router port forwarding.
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Choose the connection method that fits your session
| Method | Host availability | Network setup | Best for |
|---|---|---|---|
| Normal hosted world with Game ID | The host must remain in the world | Minimal; uses the relay path when no -port is configured |
Friends playing together casually |
| Dedicated server with Game ID | The server process can stay online independently | Minimal when using the relay path | Persistent worlds |
| Dedicated server with direct IP | The server process can stay online independently | Firewall and router port configuration are required | LAN play or situations that need direct routing |
For most groups, start with a Game ID. It avoids inbound-port configuration and is the least demanding option. The dedicated-server reference says that setting a port accepts direct connections and effectively enables cross-play; omitting -port keeps the relay-based approach. See the Core Keeper Codex dedicated-server reference for the version-specific server arguments.
What you need before connecting
- An installed and up-to-date copy of Core Keeper for every player who is joining.
- A host who can start the world, or a computer that can keep the dedicated-server process running.
- The current Game ID, or the correct direct address and port when using direct networking.
- Permission to allow the server through the host firewall and router if you choose direct IP.
Keep the game version and server package aligned. Server arguments and menus can change between releases, so use the controls shown by your current build rather than assuming that a guide for an older version has the same fields. The dedicated-server reference cited here was verified for version 1.2.1.5 on September 7, 2026.
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Join a friend-hosted world with a Game ID
This is the standard method described by the official FAQ. It does not require you to know the host’s public IP address or configure a router.
What the host does
- Start Core Keeper normally.
- Choose an existing world or create a new one, then enter the world.
- Open the pause menu.
- Copy the displayed Game ID and send it to the players who should join. Send it privately unless you intend the world to be open to anyone who receives the ID.
- Remain in the world while the others play. A normal hosted session is tied to the host’s running game.
What each joining player does
- Launch Core Keeper.
- Choose Join Game from the main menu.
- Paste or type the host’s Game ID exactly.
- Confirm the join request and wait for the world to load.
The official wording in the Core Keeper team’s March 23, 2022 Steam FAQ announcement is: “Other players should boot the game, select ‘Join Game’ in the main menu, and then enter the game ID to play in online multiplayer.”
What happens when the host leaves
If the host closes the game or leaves the hosted world, the session is no longer available through that normal game instance. Players who need the world to remain online while the owner is away should use a dedicated server instead. A laptop going to sleep, a game crash, or a lost network connection has the same practical result: other players cannot reconnect until the host starts the world again and provides a usable ID.
Sharing the ID safely
Treat a Game ID like an invitation rather than a permanent public address. Send it in a private group message and provide it again if the host restarts and receives a different ID. Do not post the ID, a server password, or a public address in an open forum unless you deliberately want unknown players to attempt to join.
Run a persistent dedicated server with a Game ID
A dedicated server is a separate application rather than a world hosted inside a player’s regular game. It can continue running on a computer or server while no one is playing locally. This is the practical choice for a shared world that friends can visit at different times.
Install the server application
- Open Steam and go to the Library.
- Change the library filter to show Tools.
- Install Core Keeper Dedicated Server. The Steam community FAQ, edited June 22, 2026, also documents installation through SteamCMD for administrators who manage a machine without the normal Steam library interface.
SteamCMD is useful on a headless machine, but its download location, app installation command, service wrapper, and launch syntax depend on the operating system. Use the package’s own files and the current Steam community instructions instead of copying a Windows desktop shortcut to a Linux host. The server tool and the game client should be kept updated together so that a client is not trying to join a server running a materially different build.
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- Launch the dedicated-server application.
- Wait for the server window or console to finish starting and generate a Game ID. Do not copy a value while the process is still printing startup messages.
- Copy that Game ID and give it to the players who should connect.
- Leave the server process running. Closing the console stops the server and makes the ID unavailable.
The machine does not need to be the same computer on which players run the game. It does need to stay powered, connected, and awake. If the operating system suspends the machine or a hosting service stops the process, the world will disappear from the connection path until the server is started again.
Join the dedicated server
Players use the same client-side path as a normal hosted world: launch the game, select Join Game, and enter the dedicated server’s Game ID. The difference is on the hosting side: the dedicated-server process, rather than a player’s active world, supplies the session. Multiple players can therefore use the world while the owner’s normal game client is closed.
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Keep the process manageable
Run the server in a location where its console can be monitored. Note the startup messages and the ID in a private administrator note, but always verify the ID printed by the current process after a restart. If you want automatic restarts or a service that starts with the operating system, use your operating system’s service or task tools; the exact configuration is platform-specific and is not a substitute for checking that the Core Keeper process actually reached a ready state.
Stop the server cleanly
When you are finished, use q in the server command window to quit. Avoid killing the process while it is writing world data unless you have no alternative. A clean quit gives the application an opportunity to close its world files normally.
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Direct networking is useful for a same-LAN server or a setup where you specifically need a routable address. It requires more configuration than a Game ID. The dedicated-server documentation states that setting -port makes the server accept direct connections and effectively enables cross-play.
Launch arguments
A conceptual command from the documented argument model is:
CoreKeeperServer -port 27015 -ip 0.0.0.0The exact executable name, launcher syntax, working directory, and additional arguments vary by the installed server package and operating system. Treat the command above as a model for the two network arguments, not as a universal copy-and-paste launcher.
-port: selects the port on which the server accepts direct traffic. The documentation uses27015as an example.-ip: selects the address to which the server binds, but it has an effect only when-portis also set.- No
-ipvalue: the reference says the server binds to0.0.0.0when the argument is omitted, allowing it to listen on the machine’s available interfaces. - No
-port: do not expect direct-IP behavior; use the Game ID and relay path instead.
Pick a port that is not already occupied by another service. If the server exits or reports that the port is unavailable, identify the conflicting application or select another port, then use that same number in the firewall, router rule, and client connection details.
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- Give the server machine a stable local address, such as a reserved DHCP address, so the address does not change unexpectedly.
- Launch the server with
-portand, if you need a specific interface,-ip. - Allow the selected port through the host computer’s firewall for the server application.
- On the same local network, have clients use the server’s local address and the configured port through the direct-connection method supported by their current game build.
The server reference specifically gives -port 27015 as an example for a same-LAN connection. It does not define one universal in-game IP-entry screen for every build, so do not assume that the Game ID box accepts an address such as 192.168.1.20:27015. Use the connection controls and client instructions shipped with the version you are running.
Test from a second device on the same network before attempting an internet connection. A successful LAN test confirms that the server is listening and that the local firewall rule is plausible; it does not prove that an internet router is forwarding traffic correctly.
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Internet setup
- Choose the listening port and configure the host firewall to allow inbound traffic for the dedicated-server process on that port.
- Log in to the router that protects the server machine.
- Create a port-forwarding rule from the chosen external port to the server machine’s local IP address and the same internal port.
- Make sure the port in the client’s connection details matches the server’s
-portvalue and the router rule. - Give players the public address and port through a private channel. Do not publish a server’s Game ID, address, or password in a public post unless you intentionally want an open server.
Keep the destination machine’s local address reserved. If DHCP later assigns a different address, the router may continue forwarding to the old computer and outside players will see a timeout even though the server is running.
Port forwarding is not needed for the normal Game ID relay method. If you do not need direct routing, remove -port and return to the Game ID workflow. This is often the quickest way to isolate whether a failure is caused by the game server or by the home network.
Game ID format and validation rules
The versioned server reference, verified for Core Keeper version 1.2.1.5, specifies that a Game ID must contain 15 to 28 alphanumeric characters. It may not contain Y, y, x, 0, or O. The exclusions make commonly confused characters easier to avoid when an ID is read from a screen.
If you manually supply an ID that breaks those rules, the server generates a random ID instead. That can make it appear that the ID you configured was ignored. Copy the ID actually printed by the running server rather than relying on a value in a launch script.
Safe handling of an ID
- Copy and paste whenever possible; do not retype a long ID from a screenshot.
- Check the character count if a join attempt fails immediately.
- Look for the forbidden characters, especially the difference between the numeral
0and the letterO. - Send the ID only to intended players. Anyone who has a valid ID may be able to attempt a connection.
- Ask the host to copy the ID after every server restart. A previous run’s identifier should not be assumed to remain active.
Cost, hardware, and hosting choices
The Game ID relay method requires no router purchase or special network accessory. A normal hosted world uses the host’s existing game computer, while a dedicated server uses another computer, a spare machine, or a managed hosting service that can keep the process online.
A self-hosted dedicated server shifts the cost from a provider to the operator: the machine must remain powered and connected, and someone must perform updates, monitor the console, and handle firewall changes. A managed Core Keeper host can remove much of that administration, but pricing, storage limits, player slots, backups, and cancellation terms vary by provider. Compare those details directly rather than assuming that every plan offers the same server build or Game ID behavior.
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1. Confirm that the host is still running
For a normal hosted world, verify that the host is inside the world, not at the title screen or desktop. For a dedicated server, verify that its process and console are still open and have not exited after an error. A correct ID cannot connect to a process that has stopped.
2. Copy a fresh Game ID
Ask the host to copy the current ID again. Check that it has 15–28 alphanumeric characters and contains none of Y, y, x, 0, or O. A stale ID from an earlier server run may no longer point to the active process.
3. Use the correct client menu
The normal client path is the main menu’s Join Game option. Make sure the player is not trying to enter a Game ID in a different multiplayer or invite field, and do not substitute an IP address when the host gave you a relay ID.
4. Identify whether the server is relay-based or direct
If the server was launched without -port, use its Game ID. If it was launched with -port, confirm that everyone is following the direct-connection details for that server. Mixing the two methods is a common cause of a connection attempt reaching the wrong endpoint.
5. Check direct-server arguments
For direct play, verify that the server really started with -port. If a particular bind address is required, verify -ip as well; remember that -ip does nothing unless -port is set. Confirm that the client is using the same port number shown in the launch configuration.
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6. Check the firewall
The host operating system may have accepted the server process but blocked inbound traffic on the selected port. Create or enable an inbound rule for that application and port, then restart the server so it binds again after the rule is active.
7. Check the router forwarding rule
For players outside the host’s LAN, the router must forward the external port to the correct internal machine and port. Verify that the destination local IP has not changed and that the forwarding settings match the server’s requirements. A rule aimed at an old DHCP address will silently send traffic to the wrong computer.
8. Test the path from the simplest location
If possible, test two clients on the same LAN first, then test from an outside network. A LAN success with an internet failure points toward the router, public address, or upstream network rather than the server world itself. Do not use a local address such as 192.168.1.20 for a player who is connecting from outside that LAN.
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9. Fall back to the relay method
If direct routing is not essential, stop the direct server, remove -port, restart it, and distribute the newly generated Game ID. This removes the inbound firewall and router variables from the connection path.
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Operational checklist for a reliable server
- Choose normal hosting for an occasional session where the owner will stay online.
- Choose the dedicated-server tool when the world must remain available independently of a player’s game client.
- Record the current Game ID only after the server has finished starting.
- Keep the server console running for the entire play session.
- Use a stable local address for a direct-IP server so router forwarding continues to target the right machine.
- Open only the port you intend to use, and share the address, Game ID, and password privately.
- Use an Ethernet connection for the server machine if unreliable Wi-Fi is causing local packet loss; wired networking is optional for relay connections, not a requirement.
- Keep the game client and dedicated-server package on compatible versions.
- When shutting down a dedicated server, enter
qin its command window.
Which method should you use?
Use a normal hosted world and Game ID if you want the fastest setup and the host will remain in the world. Use the dedicated server and Game ID if the world should persist while the owner is away without exposing an inbound port. Use direct IP and port only when LAN routing or a specific direct network path justifies configuring the firewall and router. In every case, first verify which process is hosting the world, then give players the connection information produced by that process.
Frequently Asked Questions
How do I join a Core Keeper server with a Game ID?
Launch Core Keeper, select Join Game on the main menu, and enter the Game ID supplied by the host. The host must keep the normal world or dedicated-server process running.
How do I use a Core Keeper Game ID?
The host copies the ID from the pause menu in a hosted world or from the dedicated-server console. Joining players paste or type it into Join Game. Copy the current ID again after a server restart.
Does the host have to stay online?
Yes for a normal friend-hosted world: the host must remain inside the world. No for a dedicated server, provided its separate server process remains running.
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How do I connect to a Core Keeper dedicated server?
Install Core Keeper Dedicated Server from Steam’s Tools section, launch it, wait for its Game ID, and enter that ID through Join Game. Leave the server console open while players are connected.
Can I join by IP and port?
Yes, when the dedicated server is launched with -port. Open that port in the host firewall and forward it on the router for internet play. The exact client IP-entry controls depend on the current game build.
Why does my manually chosen Game ID change?
The server reference requires 15–28 alphanumeric characters and forbids Y, y, x, 0, and O. If a supplied ID violates those rules, the server generates a random ID instead.
Do I need port forwarding for a Game ID server?
No. When -port is omitted, the Game ID and Steam relay path are the low-configuration option. Port forwarding applies to direct IP connections.
What should I do if the connection suddenly stops working?
Confirm the host or dedicated process is still running, obtain a fresh Game ID, and check that players are using the correct connection method. For direct IP, then verify -port, the firewall rule, the router forwarding destination, and the selected port.
The Bottom Line
For nearly every group, start with the Game ID workflow: it is simple, avoids router configuration, and works for both normal hosted worlds and dedicated servers. Choose a dedicated server when the world must stay available without the owner’s game client. Add -port and configure firewall and router access only when you specifically need direct LAN or internet routing.
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