As IPv6 has matured and people have gained operational experience, various co-existence and transition techniques have either been shown to be unsatisfactory or have simply been overtaken by events. This section simply lists such techniques, with minimal explanation. Readers are advised to ignore these techniques for new deployments, and to consider removing them from existing deployments.
Tunneling IPv6 over IPv4, or the converse, remains fundamental to co-existence, although various specific tunnel mechanisms are listed below as obsolete.
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Transmission of IPv6 over IPv4 Domains without Explicit Tunnels [RFC2529]. As far as is known, this was never deployed in practice.
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IPv6 Tunnel Broker [RFC3053].
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Connection of IPv6 Domains via IPv4 Clouds ("6to4") [RFC3056] [RFC3068]. The problems with this are documented in RFC6343 and it was largely deprecated by RFC7526.
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Teredo: Tunneling IPv6 over UDP through Network Address Translations (NATs) [RFC4380].
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SOCKS-based IPv6/IPv4 Gateway [RFC3089].
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ISATAP [RFC5214].
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6rd [RFC5569].
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An Incremental Carrier-Grade NAT (CGN) for IPv6 Transition [RFC6264].
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6a44 [RFC6751].