For those wondering...
ALL RJ-45 Ethernet sockets are magnetically-coupled through transformers to the equipment they're connected to. It gives galvanic isolation between the cable (and whatever is at the other end) and the device (Raspi, laptop, etc), blocks any DC that the cable might pick up from damaging the sensitive transceivers, and allows PoE to work, among other things.
Traditionally the transformers were seperate components - if you look on older network cards you can see little black boxes next to the RJ45 jack. There are two transformers, one for Transmit and one for Receive, although they are typically in the same package.
For an example see Farnell part number 1636356.
Then the jack manufacturers got a clue and developed jack sockets with integrated transformers, or "magnetics". A well-known brand is MagJack.
Check out Farnell part code 1572193.
Without the transformers the Ethernet just won't work. Bit of a show-stopper.
Some have integrated passive components as well, some support Power-over-Ethernet, some have LEDs. Lots of choices, but fundamentally they _all_ have isolating transformers.