The Free On-line Dictionary of Computing (30 December 2018):

6to4

    A protocol for transitioning from IPv4 to
   IPv6.  Networks may use 6to4 (or other transitioning protocols)
   until they support native dual-stack.

   Because 6to4 is a form of tunnelling, it requires
   encapsulation by a protocol converter.  This can cause
   performance problems due to increased latency and decreased
   MTU sizes, as described in RFC 6343
   (http://tools.ietf.org/html/rfc6343).

   RFC 3056 (http://tools.ietf.org/html/rfc3056).

   (2012-12-24)