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../../protocols/ipv6.rst:117
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IPv6 addresses often contain a long sequence of bits set to ``0``. In this case, a compact notation has been defined. With this notation, `::` is used to indicate one or more groups of 16 bits blocks containing only bits set to `0`. For example,
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../../protocols/ipv6.rst:119
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``2001:db8:0:0:8:800:200c:417a`` is represented as ``2001:db8::8:800:200c:417a``
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../../protocols/ipv6.rst:120
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``ff01:0:0:0:0:0:0:101`` is represented as ``ff01::101``
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../../protocols/ipv6.rst:121
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``0:0:0:0:0:0:0:1`` is represented as ``::1``
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../../protocols/ipv6.rst:122
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``0:0:0:0:0:0:0:0`` is represented as ``::``
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../../protocols/ipv6.rst:124
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An IPv6 prefix can be represented as `address/length`, where `length` is the length of the prefix in bits. For example, the three notations below correspond to the same IPv6 prefix :
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../../protocols/ipv6.rst:126
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``2001:0db8:0000:cd30:0000:0000:0000:0000`` / ``60``
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../../protocols/ipv6.rst:127
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``2001:0db8::cd30:0:0:0:0`` / ``60``
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../../protocols/ipv6.rst:128
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``2001:0db8:0:cd30::`` / ``60``
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../../protocols/ipv6.rst:130
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IPv6 supports unicast, multicast and anycast addresses. An IPv6 unicast address is used to identify one datalink-layer interface on a host. If a host has several datalink layer interfaces (e.g. an Ethernet interface and a WiFi interface), then it needs several IPv6 addresses. In general, an IPv6 unicast address is structured as shown in the figure below.
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../../protocols/ipv6.rst:153
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An IPv6 unicast address is composed of three parts :
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../../protocols/ipv6.rst:155
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A `global routing prefix` that is assigned to the Internet Service Provider that owns this block of addresses
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../../protocols/ipv6.rst:156
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A `subnet identifier` that identifies a customer of the ISP
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../../protocols/ipv6.rst:157
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An `interface identifier` that identifies a particular interface on a host
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../../protocols/ipv6.rst:159
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The subnet identifier plays a key role in the scalability of network layer addressing architecture. An important point to be defined in a network layer protocol is the allocation of the network layer addresses. A naive allocation scheme would be to provide an address to each host when the host is attached to the Internet on a first come first served basis. With this solution, a host in Belgium could have address ``2001:db8::1`` while another host located in Africa would use address ``2001:db8::2``. Unfortunately, this would force all routers on the Internet to maintain one route towards each host. In the network layer, scalability is often a function of the number of routes stored on the router. A network will usually work better if its routers store fewer routes and network administrators usually try to minimize the number of routes that are known by their routers. For this, they often divide their network prefix in smaller blocks. For example, consider a company with three campuses, a large one and two smaller ones. The network administrator would probably divide his block of addresses as follows :
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../../protocols/ipv6.rst:161
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the bottom half is used for the large campus
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../../protocols/ipv6.rst:162
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the top half is divided in two smaller blocks, one for each small campus
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../../protocols/ipv6.rst:164
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Inside each campus, the same division can be done, for example on a per building basis, starting from the buildings that host the largest number of nodes, e.g. the company datacenter. In each building, the same division can be done on a per floor basis, ... The advantage of such a hierarchical allocation of the addresses is that the routers in the large campus only need one route to reach a router in the smaller campus. The routers in the large campus would know more routes about the buildings in their campus, but they do not need to know the details of the organization of each smaller campus.
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../../protocols/ipv6.rst:166
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To preserve the scalability of the routing system, it is important to minimize the number of routes that are stored on each router. A router cannot store and maintain one route for each of the almost 1 billion hosts that are connected to today's Internet. Routers should only maintain routes towards blocks of addresses and not towards individual hosts. For this, hosts are grouped in `subnets` based on their location in the network. A typical subnet groups all the hosts that are part of the same enterprise. An enterprise network is usually composed of several LANs interconnected by routers. A small block of addresses from the Enterprise's block is usually assigned to each LAN.
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../../protocols/ipv6.rst:168
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In today's deployments, interface identifiers are always 64 bits wide. This implies that while there are :math:`2^{128}` different IPv6 addresses, they must be grouped in :math:`2^{64}` subnets. This could appear as a waste of resources, however using 64 bits for the host identifier allows IPv6 addresses to be auto-configured and also provides some benefits from a security point of view, as explained in section ICMPv6_.
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