6to4
IPv6-over-IPv4 tunneling mechanism. RFC 3056 (2002). A public-IPv4 address mapped automatically to an IPv6 prefix 2002:WWXX:YYZZ::/48 (W.X.Y.Z is IPv4). Standard early-IPv6 deployment, deprecated 2015 due to reliability issues.
How: client encapsulates IPv6 packets in IPv4, sends to the nearest 6to4 relay (anycast 192.88.99.1). Relay decapsulates, forwards over native IPv6. Problem: relay quality varies wildly, asymmetric routing common, debugging nightmare. RFC 7526 (2015) deprecated 6to4 + 6rd. Modern alternative: native IPv6 (most ISPs), Hurricane Electric tunnelbroker, WireGuard-IPv6 tunnels. Historic legacy: many 2001-2010-era blogs explained 6to4 as the IPv6 onramp; today mostly a historic curiosity.