Which of these statements best describes the major difference between an IPv4-compatible tunnel
and a 6to4 tunnel?

A.
An IPv4-compatible tunnel is a static tunnel, but an 6to4 tunnel is a semiautomatic tunnel.

B.
The deployment of a IPv4-compatible tunnel requires a special code on the edge routers, but a
6to4 tunnel does not require any special code.

C.
An IPv4-compatible tunnel is typically used only between two IPv6 domains, but a 6to4 tunnel is
used to connect to connect two or more IPv6 domains.

D.
For an IPv4-compatible tunnel, the ISP assigns only IPv4 addresses for each domain, but for a
6to4 tunnel, the ISP assigns only IPv6 addresses for each domain.

Explanation:

Automatic 6to4 Tunnels
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 multi-access (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 IOS 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. Automatic IPv4-Compatible IPv6 Tunnels
Automatic IPv4-compatible tunnels use IPv4-compatible IPv6 addresses. IPv4-compatible IPv6
addresses are IPv6 unicast addresses that have zeros in the high-order 96 bits of the address,
and an IPv4 address in the low-order 32 bits. They can be written as 0:0:0:0:0:0:A.B.C.D or
::A.B.C.D, where “A.B.C.D” represents the embedded IPv4 address.
The tunnel destination is automatically determined by the IPv4 address in the low-order 32 bits of
IPv4- compatible IPv6 addresses. The host or router at each end of an IPv4-compatible tunnel
must support both the IPv4 and IPv6 protocol stacks. IPv4-compatible tunnels can be configured
between border- routers or between a border-router and a host. Using IPv4-compatible tunnels is
an easy method to create tunnels for IPv6 over IPv4, but the technique does not scale for large
networks.