You must manually permit EU4 through your Windows Firewall to allow network traffic to reach your computer. Risks and Limitations

Every player must have the exact same game version and files. If even one file differs, the "checksum" will not match, and the game will prevent connection.

If players are on the same physical Wi-Fi network, they can host a "LAN" game from the multiplayer menu. This is the most stable method because it avoids external verification servers.

In some versions, the host can share their public IP address with others. This requires the host to port forward their router (typically port 1630-1640) to allow incoming traffic. Critical Technical Requirements