Discrete Ethernet magnetics
You don't need to use a integrated transformer package, but these are convenient and small for most designs.  They are also at the right impedance and are usually 1:1 but different ratios can be found for different voltage systems on the board side.

These transformers often also have baluns integrated on the network side of the two differential pairs.  Those aren't necessary according to the ethernet standard, but you may have EMI problems without them.

AT the minimum, you have to provide the two pulse transformers with the right frequency and impedance characteristics somehow.