External Cable TV/ADSL modems usually connect to the
computer through Ethernet, though they usually require tunneling.
ADSL/CableTV link through USB
Sometimes, the modem is connected to the computer using USB.
In Israel, the Cable TV USB modem is supported by Linux out of the box, using the usbnet module.
The ADSL modems are more problematic, and there are only pre-alpha
drivers for them, so be sure to ask your dealer or an Ethernet Modem (and not
a USB one).
(the USB ADSL modems are known to be problematic under Windows as well, and
should be avoide.)