Practical guide
Private IPv4 ranges: RFC 1918 reference
Reference the three RFC 1918 private IPv4 blocks, their CIDR ranges and address counts, and distinguish private from public addresses.
Updated
RFC 1918 reserves three IPv4 blocks for private internets. They are not globally unique and are not routed across the public internet.
| CIDR block | Address range | Total addresses |
|---|---|---|
10.0.0.0/8 |
10.0.0.0–10.255.255.255 |
16,777,216 |
172.16.0.0/12 |
172.16.0.0–172.31.255.255 |
1,048,576 |
192.168.0.0/16 |
192.168.0.0–192.168.255.255 |
65,536 |
Common mistakes
Not every 172.x.x.x address is private: only the range from 172.16.0.0 through 172.31.255.255 is in the RFC 1918 block. Loopback 127.0.0.0/8, link-local 169.254.0.0/16, and carrier-grade NAT 100.64.0.0/10 have other purposes; they are not RFC 1918 private space.
Use the CIDR calculator to inspect a block boundary and the CIDR subnetting guide to understand how the mask is applied.
Source
The authoritative block definitions are in RFC 1918.