6to4

[1] 6to4 is especially relevant during the initial phases of deployment to full, native IPv6 connectivity, since IPv6 is not required on nodes between the host and the destination.

If the host is configured to forward packets for other clients, often a local network, it is then a router.

Since the IPv6 hosts using autoconfiguration already have determined the unique 64 bit host portion of their address, they must simply wait for a Router Advertisement indicating the first 64 bits of prefix to have a complete IPv6 address.

Due to the high levels of misconfigured hosts and poor performance observed, an advisory about how 6to4 should be deployed was published in August 2011.

[2] Due to unsolvable operational problems using the 6to4 anycast prefix, that part of the standard was deprecated in 2015.

Note that using a reserved IPv4 address, such as those provided by RFC 1918, is undefined, since these networks are disallowed from being routed on the public Internet.

To avoid the need for users to set this up manually, the anycast address 192.88.99.1 used to be allocated.

[2][3] Packets from the IPv6 Internet to 6to4 systems must be sent to a 6to4 relay router by normal IPv6 routing methods.

A variant called IPv6 rapid deployment ("6rd") uses the same basic principles as 6to4 but uses a relay operated by the 6rd user's ISP for traffic in both directions.

According to RFC 3964, 6to4 routers and relays should ensure that: 6to4 has been default disabled since the Anniversary Update.