If you are running your Sponge server from your home, it is necessary to set up Port Forwarding on your modem or router in order for other people to connect.
Ensure you take the necessary precautions when port forwarding, because it can be insecure.
Minecraft, and thus Sponge, uses port 25565 by default. Therefore, port 25565 must be port forwarded to the internal IP address of your computer. UDP and TCP are the protocols that must be forwarded.
You may change which port is used by editing the appropriate key in your server.properties.
Port Forwarding can be performed through your router’s administrator panel. If you do not know how to navigate to your router’s administration panel, conduct an internet search for specific instructions on port forwarding for your router. The instructions are typically different for every router.
If it is preferable to have your server online upon starting up, you may need to port forward your hardware.