There isn’t a single “IPv6 version” of every IPv4 address on the public Internet. What people usually want is an embedding: IPv4-mapped (::ffff:…) for dual-stack APIs, or a 6to4 prefix for transition tech.

Paste an IPv4 address to generate those forms. Paste an IPv6 address to extract an embedded IPv4 when the address is mapped, compatible, or 6to4 — otherwise we’ll tell you it doesn’t embed one.