Subnet and PortsThe three private blocks
RFC 1918 reserves three ranges for private networks: 10.0.0.0 to 10.255.255.255 (10.0.0.0/8), 172.16.0.0 to 172.31.255.255 (172.16.0.0/12) and 192.168.0.0 to 192.168.255.255 (192.168.0.0/16). Anyone may use them inside their own network without asking, and they are never assigned to anybody on the public internet, so the same addresses are used in millions of separate networks.
Other blocks in the table
100.64.0.0/10 is shared address space for provider-side NAT. 169.254.0.0/16 is link-local: devices give themselves such an address when no server answers. 127.0.0.0/8 is loopback: addresses that always mean "this computer". The three TEST-NET blocks are for documentation and examples, and 198.18.0.0/15 is for equipment benchmarks. If you see an address from one of these in a public place, it is a sign that something is wrong or is an example.
A common slip
The middle block ends at 172.31.255.255. An address such as 172.32.0.1 is public. The subnet test tool shows this. For the explanation of how private addresses reach the internet, read private and public IP addresses.
This is an independent guide. It is not connected with IANA, the IETF or any network equipment maker. Address and port data come from the IANA registries as fetched on 14 November 2026; RFC texts were read from rfc-editor.org. Check the originals for anything you rely on.