Me (when asked what, in that case, a jack without magnetics is useful for):
It doesn’t mean no network connection at all on all devices, but this board has been designed for a magnetic jack. The magnetic bits mean better signal integrity, better filtering and shorter transmission distances for data.
This is not correct, they also crucially provide center taps to provide a path for DC currents from the transmitter to ground and to give the receive signal the correct common mode level. Also because of the aforementioned center taps the pinouts of a jack with integrated magnetics will almost certainly differer from a plain jack. http://www.smsc.com/media/Down.....12_sch.pdf
So without the correct magnetics things are unlikely to work at all.