Mobility is indisputably one of the major drives promoting IPv6. Making the transition to IPv6 from IPv4, however, is one of the major stumbling blocks. It has recently come to light that Mobile IPv6 (the IETF routing protocol for IPv6 mobile hosts) may hold some of the keys to making this transition. This paper describes mobile 4-in-6, an extension to the Mobile IPv6 protocol that allows mobile nodes to transparently communicate with the IPv4 Internet irrespective of their current location on the IPv6 Internet. Furthermore, the protocol does not require the mobile node to hold an IPv4 address other than on its home network, and does not assume the availability of any Mobile IPv4 services.