The mailing address of every device
An IP address is a unique numerical label assigned to every device on a network. It's the mailing address that tells the network where to deliver each packet. Without it, your laptop and Google's servers have no way to find each other.
IPv4 is the format you'll see most often: four numbers separated by dots, like 192.168.1.42. Each number is an octet (8 bits) and ranges from 0–255, giving us about 4.3 billion possible addresses. We ran out of those years ago — that's why IPv6 exists, but IPv4 still rules day-to-day operations.
Private vs. public — the split that runs your house
Three IPv4 ranges are reserved for private networks — your home, your office, your homelab:
| Range | CIDR | Where you'll see it |
|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | Large corporate networks, Tailscale |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | Mid-size networks, Docker default bridge |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | Most home routers |
Private IPs are not routable on the internet. Your home router has one public IP from your ISP that all your devices share via NAT (we'll cover that in lesson 7). Inside the LAN, every device gets a private IP — usually from DHCP.
IPv6 in one breath
IPv6 addresses look like 2001:0db8:85a3::8a2e:0370:7334. They're 128 bits (vs IPv4's 32), giving us 340 undecillion addresses — effectively unlimited. macOS, Linux, and modern routers dual-stack both. The tools you'll learn (ping, ssh, curl) all speak both. You just type the right format.