I want to build a device using a Raspberry Pi where you plug in the Ethernet port into any Ethernet port device and then from the Raspberry Pi it routes it back into the same device. Say for example:
Cable Modem > Raspberry Pi > Desktop
It would kind of be like a network switch than a gateway. The problem is, can you power the Raspberry Pi via the Ethernet port of these devices? Can you solder certain points?
There is a PoE solution designed specifically for the Pi. The Pi PoE Switch Hat is in production and available for presale). As this is a hat it is designed for the 40pin Pi's (A+ B+ and Pi2).
There is a video guide here on PoE for Pi0
I also saw a pair of RJ45 plug/sockets pairs with 2.1mm Power plug or socket (one of each) as CPC (at http://cpc.farnell.com )Website terms prevents providing direct item link! with a part number SR09618 (CPC is the Consumer division of Farnell, one of the Official UK RPi suppliers) - at the time of writing they were on offer for £5.33 + V.A.T. under the extended code SR0961887. These would enable you to couple power into and out of a Cat5+ cable (the instructions do call for copper-cored wires in that cable NOT copper-cased-aluminium ones) in the same way as part of yingw's answer does using some home-made adapters.
I think you will need to use special POE hubs/switches if you want a more extensive system than routing Power over more than just one cable.
You will also want to account for any voltage drop along the cable - perhaps by injecting 7 Vdc in at the source end and use a local regulator to provide the exact 5 Volt, 1 to 2 Amp supply for the Pi. (Or by using a buck/boost regulator module from a supplier on an on-line auction site, they can be sourced for only a few pounds...)
