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However, in parallel with these short-term solutions, which have allowed the IPv4 Internet to continue to be usable until now, the Internet Engineering Task Force started working on developing a replacement for IPv4. This work started with an open call for proposals, outlined in :rfc:`1550`. Several groups responded to this call with proposals for a next generation Internet Protocol (IPng) :
TUBA proposed in :rfc:`1347` and :rfc:`1561`
PIP proposed in :rfc:`1621`
SIPP proposed in :rfc:`1710`
The IETF decided to pursue the development of IPng based on the SIPP proposal. As IP version `5` was already used by the experimental ST-2 protocol defined in :rfc:`1819`, the successor of IP version 4 is IP version 6. The initial IP version 6 defined in :rfc:`1752` was designed based on the following assumptions :
IPv6 addresses are encoded as a 128 bits field
The IPv6 header has a simple format that can easily be parsed by hardware devices
A host should be able to configure its IPv6 address automatically
Security must be part of IPv6
The IPng address size
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.
IPv6 addressing architecture
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.
Textual representation of IPv6 addresses
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 :
``abcd:ef01:2345:6789:abcd:ef01:2345:6789``
``2001:db8:0:0:8:800:200c:417a``
``fe80:0:0:0:219:e3ff:fed7:1204``
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,
``2001:db8:0:0:8:800:200c:417a`` is represented as ``2001:db8::8:800:200c:417a``
``ff01:0:0:0:0:0:0:101`` is represented as ``ff01::101``
``0:0:0:0:0:0:0:1`` is represented as ``::1``
``0:0:0:0:0:0:0:0`` is represented as ``::``
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 :
``2001:0db8:0000:cd30:0000:0000:0000:0000`` / ``60``
``2001:0db8::cd30:0:0:0:0`` / ``60``
``2001:0db8:0:cd30::`` / ``60``
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.
An IPv6 unicast address is composed of three parts :
A `global routing prefix` that is assigned to the Internet Service Provider that owns this block of addresses
A `subnet identifier` that identifies a customer of the ISP

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locale/pot/protocols/ipv6.pot, string 27