Private & Public IP Classifier

Triage starts with a question that sounds trivial: is this address ours, or somebody's on the internet? It is answered wrong often enough that the mistakes have names.

172.32.0.1 is public

The RFC 1918 middle block is 172.16.0.0/12, which ends at 172.31.255.255. It looks like it should run to 172.32, and a hand-written check that assumes so will treat an internet address as internal. This is the single most common error in a hand-rolled classifier.

There are more than three private ranges

100.64.0.0/10 is Shared Address Space from RFC 6598, used inside carrier-grade NAT. It is neither RFC 1918 private nor globally routable, and a check that only knows the three familiar blocks will call it public — and then treat a carrier-internal address as an external one. 169.254.0.0/16 is link-local, which usually means DHCP failed rather than anything routed. The documentation ranges from RFC 5737 — 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 — exist precisely so that examples never name a real host, so one appearing in live telemetry is worth a second look.

Every answer cites its range and its RFC

Matching is longest-prefix, against the IANA special-purpose registries, so the most specific block wins rather than whichever was tested first. Each result names the block it matched, which is what makes the answer checkable instead of something you have to trust.

"Public" is a statement about allocation

It means the address falls outside every special-purpose range, so it is allocatable as global unicast. Whether it is assigned, routed, or has a host answering are three further questions — and none can be answered without a network lookup this page deliberately does not make.

10.0.0.5 is Private-Use under RFC 1918, matched against 10.0.0.0/8. 100.64.1.1 is Shared Address Space under RFC 6598 — not private, not routable. 203.0.113.9 is documentation space under RFC 5737, which is why it appears throughout this site's own examples.