Q I think that my modem-router has a built-in firewall that is turned on, and protective, by default. How can I tell?
A You may get a clear answer by performing an Internet search on the name and model number of the modem-router, together with the word ‘firewall’.
If you are still unsure, access its admin interface, either using the software provided with it, or by pointing your browser at its network IP address; the latter may be in its default setting, such as 192.168.0.0.
If this does not result in display of its login screen, you may need to test the firewall out instead. You can do this from a device that is connected to the Internet via a different route, such as an iPhone or iPad operating over a cellular connection. Try pinging your Mac’s IP address to see if it responds.
You could also start web sharing, through the Sharing pane in System Preferences, and try to connect to your Mac’s IP address using HTTP, such as http://18.104.22.168. A well-configured firewall may still respond to pings (unless in ‘stealth mode’, which many prefer for security reasons), but should not allow any incoming web connections. Provided that you are certain you are using the correct IP address, you could also run a port scan.
Updated from the original, which was first published in MacUser volume 27 issue 13, 2011.