You should check out this video. They're using MQTT for their communication protocol, and POE (power-over-ethernet) modules on all their remote arduino light switches.
If you have long runs, trying to distribute 9v will run into problems. Depending on your current draw, you will suffer voltage I-R drop down the wire which if it gets bad enough will cause brownouts and flaky behavior. That's why POE uses 48v for it's distribution, with stepdown transformers at each node. With a high voltage the current is low, which results in small I-R drops in your power distribution line. Same reason power companies run high voltage lines to your local transformer, and step it down for the final leg into your house.