An automatic 6to4 tunnel allows isolated IPv6 domains to be connected over an IPv4 network to remote IPv6 networks. The key
difference between automatic 6to4 tunnels and manually configured tunnels is that the tunnel is not point-to-point; it is
point-to-multipoint. In automatic 6to4 tunnels, routers are not configured in pairs because they treat the IPv4 infrastructure
as a virtual nonbroadcast multiaccess (NBMA) link. The IPv4 address embedded in the IPv6 address is used to find the other
end of the automatic tunnel.
An automatic 6to4 tunnel may be configured on a border router in an isolated IPv6 network, which creates a tunnel on a per-packet
basis to a border router in another IPv6 network over an IPv4 infrastructure. The tunnel destination is determined by the
IPv4 address of the border router extracted from the IPv6 address that starts with the prefix 2002::/16, where the format
is 2002:border-router-IPv4-address ::/48. Following the embedded IPv4 address are 16 bits that can be used to number networks within the site. The border router
at each end of a 6to4 tunnel must support both the IPv4 and IPv6 protocol stacks. 6to4 tunnels are configured between border
routers or between a border router and a host.
The simplest deployment scenario for 6to4 tunnels is to interconnect multiple IPv6 sites, each of which has at least one
connection to a shared IPv4 network. This IPv4 network could be the global Internet or a corporate backbone. The key requirement
is that each site have a globally unique IPv4 address; the Cisco software uses this address to construct a globally unique
6to4/48 IPv6 prefix. As with other tunnel mechanisms, appropriate entries in a Domain Name System (DNS) that map between hostnames
and IP addresses for both IPv4 and IPv6 allow the applications to choose the required address.