My server is so old (2012) it has parallel port. If you connect all pins together it has enough power to operate 220V relay and thus modem and router can be turned on and off without any fancy hardware.
$ cat wifi-off
#! /usr/bin/python2
import parallel,time
p = parallel.Parallel() # open LPT1 or /dev/parport0
p.setData(0xFF)
$ cat wifi-on
#! /usr/bin/python2
import parallel,time
p = parallel.Parallel() # open LPT1 or /dev/parport0
p.setData(0x0)The easiest approach is to just use a commonly available relay modules used with Arduino or similar platforms/microcontrollers, as they already contain minimum or all protection mechanisms. So, the module will contain the reversed diode in parallel with the relay, while additional transistor and optocoupler will allow you a complete galvanic isolation of the controlling- from the and relay circuit.
YouTube has plenty of user-friendly resources with schematics and functional overviews, just search for "relay module" or "relay arduino".
Great overview though, thank you for posting that.