There is an official Power-over-Ethernet standard that allows devices and routers to activate a 48VDC power line on the unused Ethernet pins (4,5 and 7,8) but PoE is expensive, requiring a special router and then a step-down converter to handle the high 48V line

For DIY projects, you may want to consider using these cables instead. One cable (the injector half) has a Ethernet plug and port and a 2.1mm jack. When you plug in a power supply (say a 9VDC wall adapter), the injector merges the power adapter in so that pins 4 and 5 connect to the power adapter ground and pins 7 and 8 have the positive 9VDC. The other cable (the extractor half) does the opposite, splitting off pins 4,5 and 7,8 and bringing them to a 2.1mm DC barrel plug. You can then use the DC plug to power your projects as if it was connected directly to the wall adapter.