IP Address Info

You're triaging a bug report and the user pasted an IP address. You want to know fast: is it IPv4 or IPv6, public or private, well-formed or garbage? Throwing it into a WHOIS service is overkill if all you need is the basic structural answer before deciding whether to keep digging.

Drop in an address. The tool tells you the version, whether it falls in a private range (RFC 1918 for v4, ULA for v6), whether it's a loopback or link-local address, and whether the format is even valid in the first place. It's structural analysis only — no geolocation, no reverse DNS, no ASN lookup. For those you'll want a service that actually queries upstream registries; this is the first 30-second pass before that.

The parsing rules: IPv4 is four decimal octets separated by dots, each 0-255. Octets in the address can be padded with leading zeros (192.168.001.001 is technically still parseable, though some parsers reject it). IPv6 is eight 16-bit groups separated by colons, with double-colon (::) abbreviation allowed once per address to compress runs of zero. The mapped form ::ffff:1.2.3.4 carries an IPv4 address inside an IPv6 wrapper — common in dual-stack systems where an IPv6 socket also accepts v4 connections.

A worked example: paste in 10.0.5.42. Output: valid IPv4, private (RFC 1918 — the 10.0.0.0/8 block), not loopback, not link-local. So it's an internal address that won't tell you anything about a user's location. Now paste in 2001:db8::1. Output: valid IPv6, but in the documentation prefix (2001:db8::/32) which is reserved for examples and shouldn't appear in real traffic. That's a tell — if production logs show this address, someone copy-pasted from a tutorial. Final example: paste in fe80::1. Output: valid IPv6, link-local (fe80::/10) — local-only address that doesn't route between networks.

What the tool deliberately doesn't do: WHOIS, geolocation, ASN lookup, or reverse DNS. Those require querying external databases (MaxMind for geo, ARIN/RIPE/APNIC for WHOIS, your DNS resolver for reverse). The data is licensed in some cases and has rate limits in all cases, which is why a quick structural check is the right first pass before deciding whether to escalate to a paid lookup service. Anycast and CDN addresses are particularly tricky — a Cloudflare-fronted site shows up as Cloudflare's IP regardless of the user's location, so geo on the source IP tells you about the CDN edge node, not the user. Treat IP-based location data with healthy skepticism even when you do query a proper service.