RFC 1918 addresses
termnetworking
The private IPv4 ranges - 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 - routable inside, never on the public internet, and the reason NAT exists.
Published in 1996 as address exhaustion loomed; every home 192.168.0.1 descends from it.
RFC 1918 set aside three ranges for private use, and they are the addresses almost every internal network is built from. They are not routable on the public internet, which is the entire point: anyone may use them, no coordination is needed, and traffic to them cannot leak beyond a border router that is doing its job.
The consequence nobody plans for is collision. Because everyone picks from the same three ranges, and because the same handful of subnets are picked far more often than the rest, merging two networks that both chose the popular ones means renumbering something. Every acquisition, every partner VPN, and every cloud peering eventually meets this, and the organizations that avoid it are the ones that allocated deliberately from the large private range rather than accepting whatever a device shipped with.
The other trap is treating private addressing as security. A private address means unreachable from outside without translation, not protected, and once an attacker is inside the perimeter the address space offers no resistance at all. It is an addressing decision that has a routing consequence, and it should not be mistaken for a control.