Just get a DHCP client when you setup your box. You may need to send the 'hostname' as an identifier (not client-identifier) for most cablemodem ISPs. With dhcpcd client I use something like:
dhcpcd -d -h csNNNNNN eth0
the -d tells it to log verbosely and the -h tells it to send the hostname you supplied (the csNNNNNN).