IPv6 transition addresses
Build IPv4-mapped/6to4/NAT64 IPv6 forms from an IPv4 address, or decode an IPv6 address back to its embedded IPv4 (and, for Teredo, server/client/port/flags). Reports what pattern was recognised, not whether the address is reachable or in use.
IPv6 transition addresses builds the IPv4-mapped, IPv4-compatible, 6to4 and NAT64 forms from an IPv4 address, each line naming its RFC. Hand it an IPv6 address and it works the other way, recognising those four and unpacking Teredo into server, flags, port and client. Teredo is decode only: building one needs a server and port you never supply.
Questions
What does it do with an IPv4 address?
It builds every well known embedding: the IPv4-mapped form from RFC 4291, the deprecated IPv4-compatible form, the 6to4 /48 prefix from RFC 3056, and the NAT64 well known prefix form from RFC 6052. Each line names its RFC, and the dotted quad tail is kept where RFC 5952 recommends it.
What does it do with an IPv6 address?
It recognises the same schemes in reverse and prints the embedded IPv4 for whichever match. It also decodes Teredo addresses under 2001:0000, giving the server IPv4, the flags with the cone bit called out, and the deobfuscated client port and client IPv4.
Can it build a Teredo address for me?
No, decode only. Constructing one needs a Teredo server address and a client port that this tool is never given, so it reads those fields out of an existing address instead of inventing them. Give it an IPv6 address under 2001:0000 and it reports the server, flags, port and client address.
What if my address is not one of these?
It says so: none of IPv4-mapped, IPv4-compatible, 6to4, NAT64 or Teredo matched, and that the address looks native or belongs to a scheme it does not know. It never guesses a pattern to fill the space. One address can match more than one scheme, and every match is listed.
Does a match mean the address is live?
No. Everything here is pattern recognition on the bits you typed, so it reports what layout was recognised and nothing else. It does not tell you whether the address is routed, reachable or in use, and no request is made to find out. Nothing leaves your device at any point.