Most likely you are using someone else's connection, but only if you're sure that you don't have any wireless router.
It might be that you have wireless, but you don't know it. It happened to us once, and we bought a brand new router, paid it, and took a whole week to configure it and wait until our ISP granted us access to the verification server they use for unknown purposes and objectices, and then we found out a convenient "Wireless network on/off" switch in the back of our router, as well as a small antenna.