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.

RFC 1918 quick reference: prefix, mask, and size

10.0.0.0/8 uses mask 255.0.0.0, spans 10.0.0.0–10.255.255.255, and contains 16,777,216 addresses. 172.16.0.0/12 uses 255.240.0.0, spans 172.16.0.0–172.31.255.255, and contains 1,048,576 addresses. 192.168.0.0/16 uses 255.255.0.0, spans 192.168.0.0–192.168.255.255, and contains 65,536 addresses. These are total block sizes, not usable-host counts for a subnet created inside a block. RFC 1918 calls Class A, B, and C language pre-CIDR notation: the three allocations correspond respectively to one former Class A network, 16 contiguous former Class B networks, and 256 contiguous former Class C networks. The CIDR prefixes and exact ranges are the unambiguous reference.

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.

How to tell whether an IPv4 address is private

Compare the complete address with all three inclusive ranges. It is RFC 1918 private only if it is from 10.0.0.0 through 10.255.255.255, from 172.16.0.0 through 172.31.255.255, or from 192.168.0.0 through 192.168.255.255. For example, 10.42.7.9, 172.20.1.1, and 192.168.50.4 are private; 11.0.0.1, 172.32.0.1, and 192.169.0.1 are not. This classification covers the address block, not whether one address is assignable inside a particular subnet.

100.64.0.0/10 is shared address space, not RFC 1918

RFC 6598 reserves 100.64.0.0 through 100.127.255.255 (100.64.0.0/10) as Shared Address Space for service-provider uses such as carrier-grade NAT. It is not one of the three RFC 1918 private blocks, even though it is not globally reachable and may appear behind an ISP connection. Likewise, 127.0.0.0/8 is loopback and 169.254.0.0/16 is link-local. Use the IANA IPv4 Special-Purpose Address Registry when you need the exact category and properties of a non-public address.

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.