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The last type of unicast IPv6 addresses are the `Link Local Unicast` addresses. These addresses are part of the `fe80::/10` address block and are defined in :rfc:`4291`. Each host can compute its own link local address by concatenating the `fe80::/64` prefix with the 64 bits identifier of its interface. Link local addresses can be used when hosts that are attached to the same link (or local area network) need to exchange packets. They are used notably for address discovery and auto-configuration purposes. Their usage is restricted to each link and a router cannot forward a packet whose source or destination address is a link local address. Link local addresses have also been defined for IPv4 :rfc:`3927`. However, the IPv4 link local addresses are only used when a host cannot obtain a regular IPv4 address, e.g. on an isolated LAN.
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``/128`` in the rare case when it is known that no more than one host will be attached
There is one difficulty with the utilization of these IPv6 prefixes. Consider Belnet, the Belgian research ISP that has been allocated the ``2001:6a8::/32`` prefix. Universities are connected to Belnet. UCLouvain uses prefix ``2001:6a8:3080::/48`` while the University of Liege uses ``2001:6a8:2d80::/48``. A commercial ISP uses prefix ``2a02:2788::/32``. Both Belnet and the commercial ISP are connected to the global Internet.
The Belnet network advertises prefix ``2001:6a8::/32`` that includes the prefixes from both UCLouvain and ULg. These two subnetworks can be easily reached from any internet connected host. After a few years, UCLouvain decides to increase the redundancy of its Internet connectivity and buys transit service from ISP1. A direct link between UCLouvain and the commercial ISP appears on the network and UCLouvain expects to receive packets from both Belnet and the commercial ISP.
Now, consider how a router inside ``alpha.com`` would reach a host in the ``UCLouvain`` network. This router has two routes towards ``2001:6a8:3080::1``. The first one, for prefix ``2001:6a8:3080::/48`` is via the direct link between the commercial ISP and UCLouvain. The second one, for prefix ``2001:6a8::/32`` is via the Internet and Belnet. Since :rfc:`1519` when a router knows several routes towards the same destination address, it must forward packets along the route having the longest prefix length. In the case of ``2001:6a8:3080::1``, this is the route ``2001:6a8:3080::/48`` that is used to forward the packet. This forwarding rule is called the `longest prefix match` or the `more specific match`. All IP routers implement this forwarding rule.
To understand the `longest prefix match` forwarding, consider the IPv6 routing below.
With the longest match rule, the route ``::/0`` plays a particular role. As this route has a prefix length of `0` bits, it matches all destination addresses. This route is often called the `default` route.
a packet with destination ``2a02:2788:2c4:16f::1`` received by router `R` is destined to a host on interface ``eth0`` .
a packet with destination ``2001:6a8:3080::1234`` matches three routes : ``::/0``, ``2001:6a8::/32`` and ``2001:6a8:3080::/48``. The packet is forwarded via gateway ``fe80::bad:cafe``
a packet with destination ``2001:1890:123a::1:1e`` matches one route : ``::/0``. The packet is forwarded via ``fe80::dead:beef``
a packet with destination ``2001:6a8:3880:40::2`` matches two routes : ``2001:6a8::/32`` and ``::/0``. The packet is forwarded via ``fe80::aaaa:bbbb``
The longest prefix match can be implemented by using different data structures One possibility is to use a trie. Details on how to implement efficient packet forwarding algorithms may be found in [Varghese2005]_.
For the companies that want to use IPv6 without being connected to the IPv6 Internet, :rfc:`4193` defines the `Unique Local Unicast (ULA)` addresses (``fc00::/7``). These ULA addresses play a similar role as the private IPv4 addresses defined in :rfc:`1918`. However, the size of the ``fc00::/7`` address block allows ULA to be much more flexible than private IPv4 addresses.
Furthermore, the IETF has reserved some IPv6 addresses for a special usage. The two most important ones are :
``0:0:0:0:0:0:0:1`` (``::1`` in compact form) is the IPv6 loopback address. This is the address of a logical interface that is always up and running on IPv6 enabled hosts.
``0:0:0:0:0:0:0:0`` (``::`` in compact form) is the unspecified IPv6 address. This is the IPv6 address that a host can use as source address when trying to acquire an official address.
The last type of unicast IPv6 addresses are the `Link Local Unicast` addresses. These addresses are part of the `fe80::/10` address block and are defined in :rfc:`4291`. Each host can compute its own link local address by concatenating the `fe80::/64` prefix with the 64 bits identifier of its interface. Link local addresses can be used when hosts that are attached to the same link (or local area network) need to exchange packets. They are used notably for address discovery and auto-configuration purposes. Their usage is restricted to each link and a router cannot forward a packet whose source or destination address is a link local address. Link local addresses have also been defined for IPv4 :rfc:`3927`. However, the IPv4 link local addresses are only used when a host cannot obtain a regular IPv4 address, e.g. on an isolated LAN.
All IPv6 hosts have several addresses
An important consequence of the IPv6 unicast addressing architecture and the utilization of link-local addresses is that each IPv6 host has several IPv6 addresses. This implies that all IPv6 stacks must be able to handle multiple IPv6 addresses.
The addresses described above are unicast addresses. These addresses are used to identify (interfaces on) hosts and routers. They can appear as source and destination addresses in the IPv6 packets. When a host sends a packet towards a unicast address, this packet is delivered by the network to its final destination. There are situations, such as when delivering video or television signal to a large number of receivers, where it is useful to have a network that can efficiently deliver the same packet to a large number of receivers. This is the `multicast` service. A multicast service can be provided in a LAN. In this case, a multicast address identifies a set of receivers and each frame sent towards this address is delivered to all receivers in the group. Multicast can also be used in a network containing routers and hosts. In this case, a multicast address identifies also a group of receivers and the network delivers efficiently each multicast packet to all members of the group. Consider for example the network below.
Assume that ``B`` and ``D`` are part of a multicast group. If ``A`` sends a multicast packet towards this group, then ``R1`` will replicate the packet to forward it to ``R2`` and ``R3``. ``R2`` would forward the packet towards ``B``. ``R3`` would forward the packet towards ``R4`` that would deliver it to ``D``.
Finally, :rfc:`4291` defines the structure of the IPv6 multicast addresses [#fmultiiana]_. This structure is depicted in the figure below.
The low order 112 bits of an IPv6 multicast address are the group's identifier. The high order bits are used as a marker to distinguish multicast addresses from unicast addresses. Notably, the 4-bit `Flags` field indicates whether the address is temporary or permanent. Finally, the `Scope` field indicates the boundaries of the forwarding of packets destined to a particular address. A link-local scope indicates that a router should not forward a packet destined to such a multicast address. An organization local-scope indicates that a packet sent to such a multicast destination address should not leave the organization. Finally the global scope is intended for multicast groups spanning the global Internet.
Among these addresses, some are well known. For example, all hosts automatically belong to the ``ff02::1`` multicast group while all routers automatically belong to the ``ff02::2`` multicast group. A detailed discussion of IPv6 multicast is outside the scope of this chapter.
IPv6 packet format
The IPv6 packet format was heavily inspired by the packet format proposed for the SIPP protocol in :rfc:`1710`. The standard IPv6 header defined in :rfc:`2460` occupies 40 bytes and contains 8 different fields, as shown in the figure below.
The IP version 6 header (:rfc:`2460`)
Apart from the source and destination addresses, the IPv6 header contains the following fields :
`Version` : a 4 bits field set to `6` and intended to allow IP to evolve in the future if needed
`Traffic class` : this 8 bits field indicates the type of service expected by this packet and contains the ``CE`` and ``ECT`` flags that are used by `Explicit Congestion Notification`
`Flow Label` : this field was initially intended to be used to tag packets belonging to the same `flow`. A recent document, :rfc:`6437` describes some possible usages of this field, but it is too early to tell whether it will be really used.
`Payload Length` : this is the size of the packet payload in bytes. As the length is encoded as a 16 bits field, an IPv6 packet can contain up to 65535 bytes of payload.

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