IPv4 has been around since 1981. Its address space — 2^32 ≈ 4.3 billion — was fully allocated by the IANA to the regional registries between 2011 and 2019.
NAT (and later CGNAT) is how the internet keeps running on IPv4 despite the exhaustion: hundreds of devices behind one public IP via address translation.
See also
- IPv6The 128-bit successor to IPv4 — addresses look like 2001:db8::1 and there are 2^128 of them.
- CIDRA notation for IP address ranges using a prefix length, e.g. 192.168.1.0/24 = 256 addresses.
- CGNATA second layer of NAT operated by an ISP, sharing one public IP between many customers because IPv4 ran out.
- RFC 1918The IPv4 ranges set aside for private use — 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Not routed on the public internet.