Wake on LAN, machine on different subnet

[Colin Law]

I am trying to use WOL to wake up a machine on a different subnet of
my local network.  The subnet is accessible from the machine running
the wakeup command.  I realise that WOL uses UDP, which will not
normally cross the subnet boundary.  However the command wakeonlan
(installed via package etherwake) has an option -i that the docs
suggest should do what I want.  I am using

sudo wakeonlan -i 192.168.10.255   nn:nn:nn:nn:nn:nn

where 192.168.10.255 is the subnet the machine is on.  If I run
wakeonlan without the -i option from a machine on the 10.x subnet then
it does wake so the problem seems to be that -i is not doing what I
hoped.

Any suggestions as to what the problem might be?

Colin L.