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../../protocols/ipv6.rst:174
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In practice, there are several types of IPv6 unicast address. Most of the `IPv6 unicast addresses <http://www.iana.org/assignments/ipv6-address-space/ipv6-address-space.xhtml>`_ are allocated in blocks under the responsibility of IANA_. The current IPv6 allocations are part of the `2000::/3` address block. Regional Internet Registries (RIR) such as RIPE_ in Europe, ARIN_ in North-America or AfriNIC in Africa have each received a `block of IPv6 addresses <http://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xhtml>`_ that they sub-allocate to Internet Service Providers in their region. The ISPs then sub-allocate addresses to their customers.
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../../protocols/ipv6.rst:176
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When considering the allocation of IPv6 addresses, two types of address allocations are often distinguished. The RIRs allocate `provider-independent (PI)` addresses. PI addresses are usually allocated to Internet Service Providers and large companies that are connected to at least two different ISPs [CSP2009]_. Once a PI address block has been allocated to a company, this company can use its address block with the provider of its choice and change its provider at will. Internet Service Providers allocate `provider-aggregatable (PA)` address blocks from their own PI address block to their customers. A company that is connected to only one ISP should only use PA addresses. The drawback of PA addresses is that when a company using a PA address block changes its provider, it needs to change all the addresses that it uses. This can be a nightmare from an operational perspective and many companies are lobbying to obtain `PI` address blocks even if they are small and connected to a single provider. The typical size of the IPv6 address blocks are :
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../../protocols/ipv6.rst:178
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``/32`` for an Internet Service Provider
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../../protocols/ipv6.rst:179
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``/48`` for a single company
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../../protocols/ipv6.rst:180
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``/56`` for small user sites
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../../protocols/ipv6.rst:181
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``/64`` for a single user (e.g. a home user connected via ADSL)
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../../protocols/ipv6.rst:182
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``/128`` in the rare case when it is known that no more than one host will be attached
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../../protocols/ipv6.rst:189
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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.
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../../protocols/ipv6.rst:196
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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.
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../../protocols/ipv6.rst:199
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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.
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../../protocols/ipv6.rst:201
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To understand the `longest prefix match` forwarding, consider the IPv6 routing below.
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../../protocols/ipv6.rst:214
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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.
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../../protocols/ipv6.rst:216
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a packet with destination ``2a02:2788:2c4:16f::1`` received by router `R` is destined to a host on interface ``eth0`` .
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../../protocols/ipv6.rst:217
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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``
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../../protocols/ipv6.rst:218
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a packet with destination ``2001:1890:123a::1:1e`` matches one route : ``::/0``. The packet is forwarded via ``fe80::dead:beef``
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../../protocols/ipv6.rst:219
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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``
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../../protocols/ipv6.rst:225
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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]_.
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../../protocols/ipv6.rst:230
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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.
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../../protocols/ipv6.rst:234
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Furthermore, the IETF has reserved some IPv6 addresses for a special usage. The two most important ones are :
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../../protocols/ipv6.rst:236
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``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.
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