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The IPng address size
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When the work on IPng started, it was clear that 32 bits was too small to encode an IPng address and all proposals used longer addresses. However, there were many discussions about the most suitable address length. A first approach, proposed by SIPP in :rfc:`1710`, was to use 64 bit addresses. A 64 bits address space was 4 billion times larger than the IPv4 address space and, furthermore, from an implementation perspective, 64 bit CPUs were being considered and 64 bit addresses would naturally fit inside their registers. Another approach was to use an existing address format. This was the TUBA proposal (:rfc:`1347`) that reuses the ISO CLNP 20 bytes addresses. The 20 bytes addresses provided room for growth, but using ISO CLNP was not favored by the IETF partially due to political reasons, despite the fact that mature CLNP implementations were already available. 128 bits appeared to be a reasonable compromise at that time.
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IPv6 addressing architecture
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The experience of IPv4 revealed that the scalability of a network layer protocol heavily depends on its addressing architecture. The designers of IPv6 spent a lot of effort defining its addressing architecture :rfc:`3513`. All IPv6 addresses are 128 bits wide. This implies that there are :math:`340,282,366,920,938,463,463,374,607,431,768,211,456 (3.4 \times 10^{38})` different IPv6 addresses. As the surface of the Earth is about 510,072,000 :math:`km^2`, this implies that there are about :math:`6.67 \times 10^{23}` IPv6 addresses per square meter on Earth. Compared to IPv4, which offers only 8 addresses per square kilometer, this is a significant improvement on paper.
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Textual representation of IPv6 addresses
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It is sometimes necessary to write IPv6 addresses in text format, e.g. when manually configuring addresses or for documentation purposes. The preferred format for writing IPv6 addresses is ``x:x:x:x:x:x:x:x``, where the ``x`` 's are hexadecimal digits representing the eight 16-bit parts of the address. Here are a few examples of IPv6 addresses :
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``abcd:ef01:2345:6789:abcd:ef01:2345:6789``
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``2001:db8:0:0:8:800:200c:417a``
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``fe80:0:0:0:219:e3ff:fed7:1204``
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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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``2001:db8:0:0:8:800:200c:417a`` is represented as ``2001:db8::8:800:200c:417a``
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``ff01:0:0:0:0:0:0:101`` is represented as ``ff01::101``
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``0:0:0:0:0:0:0:1`` is represented as ``::1``
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``0:0:0:0:0:0:0:0`` is represented as ``::``
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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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``2001:0db8:0000:cd30:0000:0000:0000:0000`` / ``60``
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``2001:0db8::cd30:0:0:0:0`` / ``60``
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``2001:0db8:0:cd30::`` / ``60``
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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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An IPv6 unicast address is composed of three parts :
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