RFC 1918 reserves exactly three IPv4 blocks for private internets: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. They can be reused in separate private networks and are not intended to be routed across the public Internet. That addressing property does not authenticate a device, encrypt traffic, or make a network safe.
Check the complete range, not a familiar prefix
The 10/8 block spans 10.0.0.0 through 10.255.255.255. The 172.16/12 block spans only 172.16.0.0 through 172.31.255.255; addresses such as 172.15.0.1 and 172.32.0.1 are outside it. The 192.168/16 block spans 192.168.0.0 through 192.168.255.255. Matching just 172 or 192 produces incorrect classifications.
Private is not the same as special-purpose
IANA maintains a broader IPv4 Special-Purpose Address Registry with properties that differ by block. Loopback, link-local, documentation, benchmarking, and other allocations are not automatically RFC 1918 private space, and their forwarding or global-reachability properties are not identical. Consult the current IANA registry when a special-purpose classification matters; SubnetCalc keeps its private-range page deliberately limited to RFC 1918.