USB connection to cable modem with no NIC

I have an RCA cable modem (model DCM 425) that is working fine (using it now to post this message).

The back of the modem has a USB outlet. I have an old computer with no NIC that I would like to connect to the Internet. I connected the computer to the modem via a USB cable and installed the modem's drivers, but I am unable to get any response from the network this way. The computer is not even assigned an IP address.

Do I need a NIC installed in order to connect via USB to the modem? I know I need a NIC for Ethernet, but I thought that the USB drivers would suffice with no NIC. What am I doing wrong?

Thanks.

-- Brian