I (like me) you use Linux, you can use the command-line utility wakeonlan or the graphical interface GWakeonlan. Both should be easily available.
For other operating systems I suppose there are equivalent, free and simple programs.
What you must tell the program to identify what device you want to send the WOL “magic packet” to is the MAC address (physical address) of the network interface of the device. You can get the MAC in many ways: for instance, on Linux and (I think also MacOS) you simply use command ifconfig from the command line. I seem to remember that Windows provides a similar command, only needing an option like /all or something similar to provide full information on a network card.