IPv6 address tools

What this tool checks

Enter an IPv6 address, or a MAC address, and the tool works out:

• the compressed form following RFC 5952 – lower case, no leading zeros, the longest run of zero groups replaced by :: (the first one when two runs are equally long, and never a single group) – and the fully expanded form with all 32 hex digits;
• the address type, from the IANA special-purpose registry: global unicast, unique local (ULA), link-local, multicast, documentation, loopback, unspecified, IPv4-mapped, NAT64, 6to4, Teredo, discard-only, or reserved space that shouldn't appear at all;
• the IPv4 address embedded in IPv4-mapped, NAT64 and 6to4 addresses, and for Teredo the Teredo server, the client's public IPv4 address and its UDP port;
• the scope and flags of a multicast address;
• the /64 network and the interface ID (the last 64 bits), and the MAC address hidden in an EUI-64 interface ID;
• the full reverse DNS name under ip6.arpa, as used for a PTR record.

You can type addresses in any valid form: 2001:0DB8:0:0::1, with a prefix length (2001:db8::1/64), in square brackets, or with a zone index (fe80::1%eth0, which is ignored).

Give a MAC address instead (00:1a:2b:3c:4d:5e, 00-1A-2B-3C-4D-5E, 001a.2b3c.4d5e or 12 hex digits) and the tool builds the modified EUI-64 interface ID from it and puts it in fe80::/64, the link-local prefix, or in the /64 prefix you enter in the second box.

It is pure calculation: nothing is looked up on the network. For prefix ranges, host counts and splitting a prefix into subnets, follow the link to the subnet calculator.

Reading the results

A note warns you when a global address is built from a MAC address: such addresses show your hardware address to every site you visit, so the device can be tracked across networks and its maker looked up.

Common problems and fixes

An address uses EUI-64 on the public internet

Turn on stable privacy addresses (RFC 7217) and temporary addresses (RFC 8981). Current Windows, macOS, iOS and Android do this by default; on Linux:

net.ipv6.conf.default.addr_gen_mode = 2
net.ipv6.conf.default.use_tempaddr = 2

Servers with fixed addresses should use a static, chosen interface ID such as ::10 instead.

The same address is written several ways

2001:DB8:0:0::1, 2001:db8::0:1 and 2001:db8::1 are the same address. Firewall rules, allowlists and log searches that compare text miss matches. Store and compare the RFC 5952 form.

A PTR record doesn't work

The reverse name has 32 nibbles, each a single hex digit, in reverse order. Missing a zero is the usual mistake: copy the name from the results instead of typing it, and make sure the PTR sits in the zone your provider delegated to you.

fc00:: addresses as private space

Only fd00::/8 is defined for locally assigned unique local addresses. Pick a random 40-bit Global ID after fd (RFC 4193) rather than fd00::/48, so that networks don't clash when they are joined later.

FAQ