If you check your device's network settings you'll likely see something like 192.168.1.42. That's not the address this site shows you — and both are correct at the same time.
Private IPs live inside your network
Your router hands every device on your home network a private IP address, usually in the 192.168.x.x, 10.x.x.x, or 172.16–31.x.x ranges. These ranges are reserved by standard for private use and are never routed on the public internet — thousands of unrelated networks reuse the exact same private addresses without conflict, because they never leave their own network.
Public IPs are what the internet sees
Your public IP is the address assigned by your ISP to your router itself, and it's the one every external site — including this one — sees. All the devices behind your router share this single public address when they reach the outside world.
NAT is the bridge between them
Network Address Translation (NAT), running on your router, is what makes this work: it rewrites outgoing private-IP traffic to use the shared public IP, and routes replies back to the right internal device. It's also why an inbound connection to a specific device needs a port forward — NAT has no way to know which device a fresh, unsolicited connection is meant for otherwise.
Why this matters
If you're trying to reach a device on your own network from inside it, use its private IP. If you're setting up something to be reachable from outside — a game server, a security camera, a self-hosted app — you need to know your public IP and forward the right port, which you can confirm with the port checker.