Ethernet devices are usually connected to the network though small transformers. In our projects, like the web platform and #twatch, these magnetics are integrated into the ethernet jack.
Florin is working on an ethernet project where space and weight are critical. He decided to ditch the heavy magnetics and experiment with capacitive ethernet coupling. This method uses only resistors and capacitors.
National Semiconductor has an app note – Application Note 1519 – DP83848 PHYTER® Transformerless Ethernet Operation – that shows how to use cap coupling in different topologies and how to calculate the capacitors –>
http://www.national.com/an/AN/AN-1519.pdf