TCP/IP Tutorial and Technical Overview - IBM Redbooks

► If the destination is not on the same subnet but there is at least one default
router on the subnet that is IPv6-capable, or there is a route configured to an
IPv6 router for that destination, send it to that router using IPv6. Otherwise, if
the address is an IPv4-compatible address, send the packet using automatic
IPv6-over-IPv4 tunneling. Otherwise, the destination is a node with an
IPv6-only address that is connected through an IPv4-routed area, which is not
also IPv6-routed. Therefore, the destination is unreachable.
Note: The IP address must be IPv4-compatible for tunneling to be used.
Automatic tunneling cannot be used to reach IPv6-only addresses,
because they cannot be addressed using IPv4. Packets from IPv6/IPv4
nodes to IPv4-mapped addresses are not tunnelled to because they refer
to IPv4-only nodes.
These rules emphasize the use of an IPv6 router in preference to a tunnel for
three reasons:
► There is less inefficiency, because there is no encapsulating IPv4 header.
► IPv6-only features are available.
► The IPv6 routing topology will be used when it is deployed in preference to
the pre-existing IPv4 topology.
A node does not need to know whether it is attached to an IPv6-routed or an
IPv4-routed area; it will always use an IPv6 router if one is configured on its
subnet and will use tunneling if one is not (in which case it can infer that it is
attached to an IPv4-routed area).
Automatic tunneling can be either host-to-host, or it can be router-to-host. A
source host will send an IPv6 packet to an IPv6 router if possible, but that router
might not be able to do the same, and will have to perform automatic tunneling to
the destination host itself. Because of the preference for the use of IPv6 routers
rather than tunneling, the tunnel will always be as “short” as possible. However,
the tunnel will always extend all of the way to the destination host. Because IPv6
uses the same hop-by-hop routing paradigm, a host cannot determine if the
packet will eventually emerge into an IPv6-complete area before it reaches the
destination host. In order to use a tunnel that does not extend all of the way to
the recipient, configured tunneling must be used.
The mechanism used for automatic tunneling is very simple:
1. The encapsulating IPv4 datagram uses the low-order 32 bits of the IPv6
source and destination addresses to create the equivalent IPv4 addresses
and sets the protocol number to 41 (IPv6).
382 TCP/IP Tutorial and Technical Overview