Why your friends can't join your game server

You set up the server. It starts. You join it, walk around, everything's fine. You paste the address into the group chat and wait.

"Can't connect."

If you've hosted anything from home, you've had this exact evening. And the frustrating part is that nothing about your server is broken. The thing in the way is your home network, which was built on purpose to keep strangers out. Your friends are strangers, as far as it's concerned.

It works for you. That's the trap.

When you join your own server, you're inside your network, so you never go through the part that's blocking everybody else. Even typing your public address from inside your house often lies to you: plenty of routers can't loop traffic back in to themselves, so it fails for you even when it would work for a friend. That sends people chasing problems they don't have.

Test from outside. Turn off Wi-Fi on your phone and try from mobile data, or ask one friend to be your guinea pig. Anything you test from the same network is testing a different path.

What's actually in the way

Nearly every "can't connect" comes down to one of these, roughly in order of how often they bite:

• NAT. Every home router does it. One public address, a whole house of devices behind it, and no idea where to send a connection nobody asked for. What NAT is.
• No port forward, or a wrong one. The rule that tells your router "anything on this port goes to that machine". Missing, pointed at the wrong address, or set to TCP when the game speaks UDP. Port forwarding.
• A firewall on the server machine. Windows asks the first time a server starts listening. Click the wrong thing (or click nothing) and it quietly blocks every incoming connection forever.
• CGNAT. Your ISP never gave you a public address at all. You share one with other customers, behind a NAT of theirs you can't configure. CGNAT.
• Double NAT. Two routers in a row, usually the ISP's box and one you bought. A forward on one of them isn't enough. Double NAT.

Work out which one you've got

You need two addresses: the one your router thinks it has, and the one the internet actually sees. If they're the same, you're in the easy case. If they're not, the first number tells you why.

What fixes what

Once you know the wall, the options are pretty short:

• Forward the port. Free, and the right answer when your router has the public address. Needs access to the router, and a bit of patience.
• Fix the double NAT. Put the ISP's box in bridge mode or IP passthrough so your own router gets the public address, then forward the port there.
• Ask your ISP for a public address. The only real fix for CGNAT. Some ISPs do it free if you ask, some charge for a static IP, some won't do it at all.
• Put everyone on a VPN. Tailscale, ZeroTier, Hamachi and friends all work through any of these walls. The catch is that every single player has to install it and join your network, every time someone new wants in.
• Rent a server. Moves the problem somewhere with a real address. Costs money every month, and your world lives on somebody else's machine.
• Use a relay. Your server dials out to a machine that has a public address, and players connect there. Nothing to open, nothing for players to install. That's what Void Sluice does, and it costs a little ping for the extra hop.

None of these is the one true answer. If you can forward a port, forward the port. The rest exist because a lot of people can't, and that's not their fault.