What is '::' (the double colon)?

It elides one or more consecutive groups of zeros. It can appear at most once per address. The expanded form fills all 8 groups like 0000:0000:..., and the compressed form replaces the longest run of zero groups with '::' (RFC 5952).

What's the difference between IPv4-mapped and IPv4-compatible?

IPv4-mapped (::ffff:0:0/96) is the still-current form used to represent an IPv4-only host inside an IPv6 socket. IPv4-compatible (::/96) is a deprecated early-transition format. This tool labels them separately with a badge.

How is reverse DNS (ip6.arpa) used?

DNS PTR records use this name to resolve an address back to a host name. Each nibble of the address appears in reverse, then '.ip6.arpa' is appended. For 2001:db8::1 the PTR name is '1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa'.

How are zone IDs (%eth0) and [::1]:8080 handled?

Zone IDs (RFC 4007 / 6874) are link-local interface selectors used with fe80::/10 addresses. The parser accepts them and reports the zone in a separate row. Bracketed [addr]:port is the URL convention — the address portion is parsed and the port is ignored.

What about IPv4 CIDR calculations?

This tool focuses on a single IPv6 address. For IPv4 subnet math (network/broadcast/host count) use the ip-cidr-calc tool.

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IPv6 formatter — compress / expand / reverse DNS / classify

Parse an IPv6 address (::1, 2001:db8::1, fe80::1%eth0, [::1]:8080, etc.) and view the RFC 5952 compressed form, the fully expanded 8-group form, the ip6.arpa reverse DNS name, hex-concatenated, and binary representations side by side. Auto-classifies loopback, link-local (fe80::/10), unique-local (fc00::/7), multicast (ff00::/8), global unicast (2000::/3), IPv4-mapped (::ffff:0:0/96), documentation (2001:db8::/32) and more. IPv4-mapped addresses are extracted to dotted-quad. Supports zone IDs and bracketed forms. Everything is processed in your browser.

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How to use

Type a single IPv6 address and see its RFC 5952 compressed form, the fully expanded 8-group form, the ip6.arpa reverse DNS name, hex-concatenated and binary representations side by side. The category (loopback / link-local / multicast / IPv4-mapped / global unicast etc.) is detected and shown as a badge. Each row has a one-click copy button, so you can pick the form you need for config files, DNS PTR records, or log normalization. Zone IDs (%eth0) and bracketed [::1]:8080 are accepted.

FAQ

What is '::' (the double colon)?: It elides one or more consecutive groups of zeros. It can appear at most once per address. The expanded form fills all 8 groups like 0000:0000:..., and the compressed form replaces the longest run of zero groups with '::' (RFC 5952).
What's the difference between IPv4-mapped and IPv4-compatible?: IPv4-mapped (::ffff:0:0/96) is the still-current form used to represent an IPv4-only host inside an IPv6 socket. IPv4-compatible (::/96) is a deprecated early-transition format. This tool labels them separately with a badge.
How is reverse DNS (ip6.arpa) used?: DNS PTR records use this name to resolve an address back to a host name. Each nibble of the address appears in reverse, then '.ip6.arpa' is appended. For 2001:db8::1 the PTR name is '1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa'.
How are zone IDs (%eth0) and [::1]:8080 handled?: Zone IDs (RFC 4007 / 6874) are link-local interface selectors used with fe80::/10 addresses. The parser accepts them and reports the zone in a separate row. Bracketed [addr]:port is the URL convention — the address portion is parsed and the port is ignored.
What about IPv4 CIDR calculations?: This tool focuses on a single IPv6 address. For IPv4 subnet math (network/broadcast/host count) use the ip-cidr-calc tool.

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