Operation Manual
face ID, and a 16 bit eld specifying the subnetwork ID. Again, the rest is
lled with zero bytes.
As a completely new feature introduced with IPv6, each network interface normally
gets several IP addresses, with the advantage that several networks can be accessed
through the same interface. One of these networks can be congured completely auto-
matically using the MAC and a known prex with the result that all hosts on the local
network can be reached as soon as IPv6 is enabled (using the link-local address). With
the MAC forming part of it, any IP address used in the world is unique. The only variable
parts of the address are those specifying the site topology and the public topology, de-
pending on the actual network in which the host is currently operating.
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-
tier of the home network to which it normally belongs (and the corresponding prex).
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 autoconguration 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.
21.2.3 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 21.2.2, “Address Types and Structure” (page 320)).
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
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