Note: This regulator module is not designed for use with commercial PoE systems such as Ethernet switches with 802.3af support. Commercial PoE switches can push up to 48V down the wire, which will overload the regulator module and force it to shut down. If you want to use the Freetronics Ethernet Shield with commercial 802.3af equipment you will need to use the "Power-over-Ethernet Regulator 802.3af".
The 14-24V Power-over-Ethernet regulator module mounts on the Freetronics Ethernet Shield and EtherTen to allow it to accept input voltages up to 24V. If you only intend to pass about 12V down the Ethernet cable to your shield you don't need this module: you can simply put a pair of jumpers across the PoE header pins on the Freetronics Ethernet Shield or EtherTen and let the on-board voltage regulator do all the work.
If you want to push a higher voltage down the wires (perhaps you already have a 16V, 18V, or 24V power supply lying around?) the Arduino voltage regulator will get very hot and unhappy, so this regulator module takes the power you send down the cable and pre-regulates it down to 12V before passing it through to your Arduino for the final drop to 5V.
This regulator module is designed for use with home-brew PoE injectors that typically supply 16V or 24V, not commercial PoE switches. You can buy a compatible 4 Port PoE midspan from us.