Sounds like you have hooked up your computer directly to the Internet, exposing All open ports. Could happen with fibre connections. Most cable and xDSL modems have a built-in WiFi and router with minimalistic Firewall functionality. With fibre, you have to get your own router. At least that is usually the case in my country.

Could also be your WiFi/router configured to allow incoming connections. Most equipment should be configured by default to block incoming connections.