NAT is what lets your home network full of 192.168.x.x devices share one public IP. The router maintains a translation table: "192.168.1.10:54321 ↔ 203.0.113.5:31234" for each outbound flow.
NAT broke the original end-to-end model of the internet. Modern peer-to-peer apps (video calls, BitTorrent, gaming) work around it with STUN/TURN servers and NAT-traversal techniques.
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See also
- CGNATA second layer of NAT operated by an ISP, sharing one public IP between many customers because IPv4 ran out.
- IPv4The original 32-bit IP addressing scheme — addresses look like 203.0.113.42 and there are ~4.3 billion of them.
- 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.