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Networking / LOCAL TOOL

IPv6 Address Tool

Expand, compress, and inspect IPv6 addresses, prefixes, and reverse DNS names.

Runs locally — input is never uploaded
An IPv6 address or prefix, e.g. 2001:db8::1 or 2001:db8::/48
ResultReady when you are

WHAT IT DOES

One spelling of an IPv6 address, and what the rest of it means.

An IPv6 address can be written a dozen ways and only one of them is canonical. Enter an address or a prefix to get the RFC 5952 form, the fully expanded form, the reverse DNS name under ip6.arpa, the reserved range it falls in, and -- when the address came off a network rather than out of a plan -- the MAC address its interface identifier was built from.

Why use it?

Use it to compare differently written IPv6 addresses, inspect prefix boundaries, and prepare reverse-DNS information without confusing spelling with reachability.

How to use this tool

  1. Enter an IPv6 address or prefix and inspect it; for example, 2001:0db8:0:0:0:0:0:1 normalizes to 2001:db8::1.
  2. Compare the expanded form, address classification, and any prefix or reverse-DNS fields.
  3. Treat a recovered MAC-like identifier as an interpretation of the bits, not verified hardware identity. No address is contacted or DNS record checked.

Details and limitations

  • The canonical form follows RFC 5952: lowercase, no leading zeros in a group, and the longest run of zero groups compressed once. A run of a single zero group is never compressed, because :: there saves nothing and gives the address a second legal spelling.
  • An interface identifier with ff:fe in the middle is an EUI-64 built from a MAC address, and the MAC is recovered by removing those two bytes and flipping the universal/local bit. An address like that follows the hardware from network to network, which is what privacy extensions exist to prevent.
  • A prefix reports how many /64 subnets it contains. A /64 is the unit IPv6 addressing is built on, since stateless autoconfiguration needs 64 host bits, so a prefix longer than /64 is flagged rather than counted.
  • Reverse DNS names are every nibble in reverse order under ip6.arpa. A prefix that is not a multiple of four does not land on a nibble boundary, so the zone given is the one above it, which is where a delegation can actually be cut.
  • An address in ::ffff:0:0/96 is an IPv4 address carried inside an IPv6 one, and the IPv4 address is reported separately. A log full of ::ffff: prefixes is a dual-stack socket reporting IPv4 peers, not an IPv6 deployment.
  • A zone index such as %eth0 is kept and reported rather than rejected. It names the interface a link-local address is valid on and takes no part in the arithmetic.

COMMON QUESTIONS

IPv6 Tool questions

What is the correct way to write an IPv6 address?
RFC 5952 allows one form: lowercase hex, no leading zeros inside a group, and the longest run of all-zero groups replaced by :: exactly once, choosing the leftmost run when two are equally long. A single zero group is written as 0 rather than compressed. 2001:0DB8:0000:0000:0000:0000:0000:0001 is 2001:db8::1.
Why does my IPv6 address contain ff:fe?
Because the interface identifier was derived from the interface's MAC address in EUI-64 form: ff:fe is inserted in the middle of the six MAC bytes and the universal/local bit is flipped. The tool recovers the original MAC. Such an address identifies the hardware across every network it joins, which is why operating systems now default to privacy extensions instead.
Why is a /64 special?
Stateless address autoconfiguration builds the low 64 bits from the interface, so it needs exactly 64 host bits. A prefix longer than /64 breaks it and works only for point-to-point links and similar cases, and a prefix shorter than /64 is a container for /64 subnets rather than a subnet itself.
What is ip6.arpa?
The reverse DNS zone for IPv6. The name is every nibble of the address written backwards, one per label, followed by ip6.arpa -- 32 labels for a full address. Delegations are cut on nibble boundaries, so a prefix that is not a multiple of four is reported as the zone above it.