Novell LINUX ENTERPRISE DESKTOP 10 SP2 - DEPLOYMENT GUIDE 08-05-2008 Deployment Manual page 624

For a host to go back and forth between different networks, it needs at least two address-
es. One of them, the home address, not only contains the interface ID but also an iden-
tifier of the home network to which it normally belongs (and the corresponding prefix).
The home address is a static address and, as such, it does not normally change. Still,
all packets destined to the mobile host can be delivered to it, regardless of whether it
operates in the home network or somewhere outside. This is made possible by the
completely new features introduced with IPv6, such as stateless autoconfiguration and
neighbor discovery. In addition to its home address, a mobile host gets one or more
additional addresses that belong to the foreign networks where it is roaming. These are
called care-of addresses. The home network has a facility that forwards any packets
destined to the host when it is roaming outside. In an IPv6 environment, this task is
performed by the home agent, which takes all packets destined to the home address and
relays them through a tunnel. On the other hand, those packets destined to the care-of
address are directly transferred to the mobile host without any special detours.
30.2.3 Coexistence of IPv4 and IPv6
The migration of all hosts connected to the Internet from IPv4 to IPv6 is a gradual
process. Both protocols will coexist for some time to come. The coexistence on one
system is guaranteed where there is a dual stack implementation of both protocols. That
still leaves the question of how an IPv6 enabled host should communicate with an IPv4
host and how IPv6 packets should be transported by the current networks, which are
predominantly IPv4 based. The best solutions offer tunneling and compatibility addresses
(see Section 30.2.2, "Address Types and Structure" (page 604)).
IPv6 hosts that are more or less isolated in the (worldwide) IPv4 network can commu-
nicate through tunnels: IPv6 packets are encapsulated as IPv4 packets to move them
across an IPv4 network. Such a connection between two IPv4 hosts is called a tunnel.
To achieve this, packets must include the IPv6 destination address (or the corresponding
prefix) as well as the IPv4 address of the remote host at the receiving end of the tunnel.
A basic tunnel can be configured manually according to an agreement between the
hosts' administrators. This is also called static tunneling.
However, the configuration and maintenance of static tunnels is often too labor-intensive
to use them for daily communication needs. Therefore, IPv6 provides for three different
methods of dynamic tunneling:
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