Open your browser and enter your login credentials at http://fritz.box.
The integrated security diagnostics function gives you an overview of the port shares used by the FRITZ!Box for network communications. To start it, click Diagnostics and Security. A few seconds later, you’ll see which ports are open under “Port Sharing with Home Network Devices”. These ports let internet devices establish a connection on your FRITZ!Box. If you see “No port sharing configured”, that’s a good sign.
Additionally, “1. Connection, Internet” also lists the default ports that should remain open so that the FRITZ!Box can establish internet and telephony connections. Irrespective of this, you should open ideally none or just a few ports.
You will find the settings, if your box has them, by clicking Internet, Permit Access, and then Port Sharing. In the default configuration no ports should be open in the list. Under the entry “List of Ports Opened via UPnP or PCP”, you can see which ports have been opened. Click the red cross next to the port to close it. Then click Apply to save your changes.
Important: Before removing any suspicious-looking port shares, first backup your FRITZ!Box’s settings via Wizards and Save and Restore Settings. If a network device stops working after closing a port, simply restore your former settings.