IPv4
In plain terms
The fourth version of IP, using 32-bit addresses.
Going deeper
IPv4 addresses devices with 32-bit numbers (like 192.168.1.1), providing about 4.3 billion addresses, now exhausted. It's still the dominant addressing scheme.
Under the hood
Address scarcity drove NAT and CIDR; IPv6 is the long-term successor.
Examples
192.168.1.1, home/office networks, NAT, the classic internet addressing.
Think of it like...
Like a phone-number system that ran out of unique numbers as the world grew.