Warning: This blog is about powering your raspberry pi-3/4 over ethernet cable(length upto 100Meters), It uses Passive PoE mechanism where T568b wire color
Blue/Blue-White ==> +(positive) terminal of DC supply
Brown-White/Brown ==> -(negetive) terminal of DC supply
If you dont know what is a passive PoE, then dont proceed!!!, instead buy a proper PoE Hat for Raspberry pi-3.
If you are a tinkerer/hw-enthusiest/diy-hobbyist(and you know what you are doing), here is what you need,
1)PoE Injector cable(costs less than 80cents)
2)DC-DC buck converter from aliexpress that costs less than 50cents(Look for Hesai brand, 12v-24v input and 5V/3A Output).
3)female-to-female jumper wire(cut into 4 pieces) and solder as shown in the picture
4)Cover DC-DC converter in a heatshrink sleev, and connect to raspi-3 as shown.
5)Feed 12v-DC and Network-connection to the PoE Injector and connect CAT-5 cable(upto 100m) between Injector cable and Raspberry-Pi-3 as shown below