"IP directed broadcast" must be supported on the last router to the destination subnet. Since the sleeping PC's dont have IP adresses, the machines must be calles awake by broadcast that behaves like an unicast untill they reach the destination network. There the directed broadcast is handled like a proper broadcast to wake all WOL machines.
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91672-catl3-wol-vlans.html