IPv6

IPv6 is version 6 of the Internet Protocol; it was initially called IP Next Generation (IPng) when it was picked as the winner in the IETF's IPng selection process. IPv6 is intended to replace the previous standard, IPv4, which only supports up to about 4 billion (4 × 10⁹) addresses, whereas IPv6 supports up to about 3.4 × 10³⁸ (340 undecillion) addresses. This is the equivalent of 4.3 × 10²⁰ (430 quintillion) addresses per inch² (6.7 × 10¹⁷ (670 quadrillion) addresses/mm²) of the Earth's surface. It is expected that IPv4 will be supported until at least 2025, to allow time for bugs and system errors to be corrected.

The compelling reason behind the formation of IPv6 was lack of address space, especially in the heavily populated countries of Asia such as India and China. See the article IPv4 address exhaustion for more on this topic. The introduction of network address translation (NAT) has to a certain extent alleviated this problem. NAT, however, makes certain peer-to-peer applications, such as VoIP and certain multi-user games, impossible or technically difficult. Currently the big drive for IPv6 is new uses, such as mobility, quality of service, privacy extension and so on.

IPv6 is the second version of the Internet Protocol to be formally adopted for general use. (There was also an IPv5, but it was not a successor to IPv4; rather, it was an experimental flow-oriented streaming protocol, intended to support voice, video, and audio.)

The plan is for IPv6 to form the basis for future expansion of the Internet. Although IPv6 was adopted by the IETF as the successor to IPv4 over ten years ago (in 1994), worldwide IPv6 deployment as a publicly-accessible internet is still only a few percent [1] of the size of the worldwide IPv4 Internet [1].

IPv6 addressing

The most dramatic change from IPv4 to IPv6 is the length of network addresses. IPv6 addresses, as defined by RFC 2373 and RFC 2374, are 128 bits long; this corresponds to 32 hexadecimal digits, which are normally used when writing IPv6 addresses, as described in the following section.

The number of possible addresses in IPv6 is 2¹²⁸ ≈ 3.4 x 10³⁸. The number of IPv6 addresses can also be thought of as 16³² as each of the 32 hexadecimal digits can take 16 values (see combinatorics).

In many situations, IPv6 addresses are composed of two logical parts: a 64-bit network prefix, and a 64-bit host-addressing part, which is often automatically generated from the interface MAC address.

It is often argued that 128-bit addresses are overkill, and that the Internet will never need that many. It should be noted that the rationale for the 128-bit address space is not primarily to make sure that addresses never run out, but rather to ensure that routing can be handled smoothly by keeping the address space unfragmented, rather than as the current situation is with IPv4, where a great number of discrete netblocks can be, and often are, assigned to one organization.

Notation for IPv6 addresses

IPv6 addresses, which are 128 bits long, are normally written as eight groups of four hexadecimal digits. For example,

2001:0db8:85a3:08d3:1319:8a2e:0370:7334 is a valid IPv6 address. If a four-digit group is 0000, it may be omitted. For example,
2001:0db8:85a3:0000:1319:8a2e:0370:7344
is the same IPv6 address as
2001:0db8:85a3::1319:8a2e:0370:7344

Following this rule, if more than two consecutive colons result from this omission, they may be reduced to two colons, as long as there is only one group of two or more consecutive colons. Thus
2001:0DB8:0000:0000:0000:0000:1428:57ab
2001:0DB8:0000:0000:0000::1428:57ab
2001:0DB8:0:0:0:0:1428:57ab
2001:0DB8:0::0:1428:57ab
2001:0DB8::1428:57ab
are all valid and mean the same thing, but
2001::25de::cade
is invalid because it is not clear how many 0000 groups are on each side.

Leading zeros in a group can be omitted. Thus
2001:0DB8:02de::0e13
is the same thing as
2001:DB8:2de::e13

If the address is an IPv4 address in disguise, the last 32 bits may be written in decimal; thus

::ffff:192.168.89.9 is the same as ::ffff:c0a8:5909, but not the same as ::192.168.89.9 or ::c0a8:5909. The ::ffff:1.2.3.4 format is called an IPv4-mapped address. The ::1.2.3.4 format is an IPv4-compatible address.

IPv4 addresses are easily converted to IPv6 format. For instance, if the decimal IPv4 address was 135.75.43.52 (in hexadecimal, 0x874B2B34), it could be converted to 0000:0000:0000:0000:0000:0000:874B:2B34 or ::874B:2B34. Then again, one could use the hybrid notation (IPv4-compatible address), in which case the address would be ::135.75.43.52. These IPv4-compatible addresses are being deprecated, because IPv6 transition mechanisms no longer use them. The respective RFCs will reflect this shortly.

Special addresses

There are a number of addresses with special meaning in IPv6. This is a brief table of these, in CIDR notation – see the linked page for more information.

• ::/128 – the address with all zeroes is used to specify any address, and is only to be used in software.
• ::1/128 – the loopback address is a host-local address which echoes back all the packets sent to it (corresponding to 127.0.0.1 in IPv4).
• ::/96 – The IPv4-compatible address is used as a transition mechanism in dual IPv4/IPv6 networks.
• ::ffff/96 – The IPv4-mapped address is used as a transition mechanism in dual-stack hosts.
• fe80::/10 – The link-local prefix specifies that the address only is valid in the local physical link.
• fec0::/10 – The site-local prefix specifies that the address only is valid inside the local organization. Its use has been deprecated in September 2004 by RFC 3879 and future systems must not implement any support for this special type of address anymore.
• ff00::/8 – The multicast prefix is used for multicast addresses.
  

  IPv6 packet

The header is in the first 40 bytes of the packet and contains both source and destination addresses (128 bits each), as well as the version (4-bit IP version), traffic class (8 bits, Packet Priority), flow label (20 bits, QoS management), payload length (16 bits), next header (8 bits), and hop limit (8 bits, time to live). Next comes the payload, which can be up to 64k in size in standard mode, or larger with a "jumbo payload" option.

There have been two slightly different versions of IPv6. The now-obsolete initial version, described in RFC 1883, differs from the current proposed standard version, described in RFC 2460, in two fields: 4 bits have been reassigned from flow label to traffic class. All other differences are minor.

Fragmentation is handled in the host only in IPv6. In IPv6, options also move out of the standard header and are specified by the Next Header field, similar in function to IPv4's Protocol field. A handwaving example: in IPv4 one would add a Strict Source and Record Routing (SSRR) option to the IPv4 header itself in order to enforce a certain route for the packet, but in IPv6 one would make the Next Header field indicate that a Routing header comes next. The Routing header would then specify the additional routing information for the packet, and then indicate that the, for example, TCP header comes next. This is analogous to the handling of AH and ESP in IPSec for IPv4 (which applies to IPv6 as well, of course).

IPv6 and the Domain Name System

IPv6 addresses are represented in the Domain Name System by AAAA records (so-called quad-A records) for forward lookups (by analogy with A records for IPv4); reverse lookupss take place under ip6.arpa (previously ip6.int), where address space is delegated on nibble boundaries. This scheme is defined in RFC 3596.

The AAAA scheme was one of two proposals at the time the IPv6 architecture was being designed. The other proposal would have had A6 records for the forward lookup and a number of other innovations such as bit-string labels and DNAME records. It is defined in the experimental RFC 2874 and its references.

While the AAAA scheme is a simple generalisation of the IPv4 DNS, the A6 scheme was an overhaul of the DNS to be more general, and hence more complex: