How to connect old computer to cable modem

[heddratt]

hello,
our primary computer (an IMAC) and laptop (MacBook Pro) are connected wirelessly to a Comcast cable modem (ARRIS tg 862). Our old Mac G5 has been relegated to a spare bedroom on another floor. How can I connect the old computer to the cable modem without a cable? thanks much

 

[RealBeast]

A pair of powerline adapters or MOCA adapters.