Ethernet ground multilayer pcb
Ethernet signals should be routed so that they have 100 ohm differential impedance. This can be achieved by using different stackups, but usually the layer with the differential traces will be adjacent to a ground plane layer to achieve the desired impedance. As far as the transformer goes, it depends on whether it is a discrete transformer, or integrated into an RJ45 jack. With discrete transformers it is recommended to separate the ground planes between each side of the transformer to prevent a path for noise coupling. It is also common to bridge the gap between grounds with several capacitors in case they are needed. It is not necessary or desired to have separate grounds when using an RJ45 jack with integrated magnetics which simplifies the design.