My Arris Cable modem takes 115vac right into it and has its own DC power supply built into it. My Netgear WNDR3400 Cablevision Router has an external 115 VAC Plug in unit which puts out 12VDC and plugs into the Netgear router.

The important thing is to match the Power type (AC or DC) and the polarity of the plug matching the receptacle on the modem as to plus and minus pins. Of course this has been said several times here already, methinks