IPv6 specifies a new packet format, designed to minimize packet header processing by routers. Because the headers of IPv4 packets and IPv6 packets are significantly different, the two protocols are not interoperable. However, in most respects, IPv6 is a conservative extension of IPv4. Most transport and application-layer protocols need little or no change to operate over IPv6; exceptions are application protocols that embed internet-layer addresses, such as FTP and NTPv3.
Larger address space
The most important feature of IPv6 is a much larger address space than in IPv4. The length of an IPv6 address is 128 bits, compared to 32 bits in IPv4 The address space therefore supports 2128 or approximately 3.4×1038
addresses. By comparison, this amounts to approximately 5×1028
addresses for each of the 6.8 billion people alive in 2010. (In addition, the IPv4 address space is poorly allocated, with approximately 14% of all available addresses utilized.) While these numbers are very large, it was not the intent of the designers of the IPv6 address space to assure geographical saturation with usable addresses. Rather, the longer addresses simplify allocation of addresses, enable efficient route aggregation, and allow implementation of special addressing features. In IPv4, complex Classless Inter-Domain Routing (CIDR) methods were developed to make the best use of the small address space. The standard size of a subnet in IPv6 is 264 addresses, the square of the size of the entire IPv4 address space. Thus, actual address space utilization rates will be small in IPv6, but network management and routing efficiency is improved by the large subnet space and hierarchical route aggregation.
Renumbering an existing network for a new connectivity provider with different routing prefixes is a major effort with IPv4.] With IPv6, however, changing the prefix announced by a few routers can in principle renumber an entire network since the host identifiers (the least-significant 64 bits of an address) can be independently self-configured by a host.
IPv6 hosts can configure themselves automatically when connected to a routed IPv6 network using Internet Control Message Protocol version 6 (ICMPv6) router discovery messages. When first connected to a network, a host sends a link-local router solicitation multicast request for its configuration parameters; if configured suitably, routers respond to such a request with a router advertisement packet that contains network-layer configuration parameters
If IPv6 stateless address autoconfiguration is unsuitable for an application, a network may use stateful configuration with the Dynamic Host Configuration Protocol version 6 (DHCPv6) or hosts may be configured statically.
Routers present a special case of requirements for address configuration, as they often are sources for autoconfiguration information, such as router and prefix advertisements. Stateless configuration for routers can be achieved with a special router renumbering protocol
Addressing
The most important feature of IPv6 is a much larger address space than in IPv4. IPv6 addresses are 128 bits long, compared to only 32 bits previously While the IPv4 address space contains only about 4.3×109
(4.3 billion) addresses, IPv6 supports approximately 3.4×1038
(340 undecillion) unique addresses, deemed enough for the foreseeable future.
IPv6 addresses are written in eight groups of four hexadecimal digits separated by colons, for example, 2001:0db8:85a3:0000:0000:8a2e:0370:7334. IPv6 addresses are logically divided into two parts: a 64-bit (sub-)network prefix, and a 64-bit interface identifier.
For SLAAC to work, subnets require at least a /64 address block, which is 1/(264) of total IPv6 address space. Local Internet registries get assigned at least /32 blocks, which they divide among ISPs. The obsolete RFC 3177 recommended the assignment of a /48 to end consumer sites. This was replaced by RFC 6177, which "recommends giving home sites significantly more than a single /64, but does not recommend that every home site be given a /48 either." /56s are specifically considered.
IPv6 addresses are classified by three types of networking methodologies: unicast addresses identify each network interface, anycast addresses identify a group of interfaces, usually at different locations of which the nearest one is automatically selected, and multicast addresses are used to deliver one packet to many interfaces. The broadcast method is not implemented in IPv6. Each IPv6 address has a scope, which specifies in which part of the network it is valid and unique. Some addresses are unique only on the local (sub-)network; Others are globally unique.
Some IPv6 addresses are reserved for special purposes, such as the address for loopback. Also, some address ranges are considered special, such as link-local addresses for use in the local network only, and solicited-node multicast addresses used in the Neighbor Discovery Protocol.
IPv4-mapped IPv6 addresses
Hybrid dual-stack IPv6/IPv4 implementations support a special class of addresses, the IPv4-mapped IPv6 addresses. This address type has its first 80 bits set to zero and the next 16 set to one, while its last 32 bits are filled with the IPv4 address. These addresses are commonly represented in the standard IPv6 format, but having the last 32 bits written in the customary dot-decimal notation of IPv4; for example, ::ffff:192.0.2.128 represents the IPv4 address 192.0.2.128. It substitutes the old and deprecated IPv4-compatible IPv6 address formed by ::192.0.2.128.[34]
Because of the significant internal differences between IPv4 and IPv6, some of the lower level functionality available to programmers in the IPv6 stack do not work identically with IPv4 mapped addresses. Some common IPv6 stacks do not support the IPv4-mapped address feature, either because the IPv6 and IPv4 stacks are separate implementations (e.g., Microsoft Windows 2000, XP, and Server 2003), or because of security concerns (OpenBSD) .[35] On these operating systems, it is necessary to open a separate socket for each IP protocol that is to be supported. On some systems, e.g., the Linux kernel, NetBSD, and FreeBSD, this feature is controlled by the socket option IPV6_V6ONLY as specified in RFC 3493
Address Format
IPv6 addresses have two logical parts: a 64-bit network prefix, and a 64-bit host address part. (The host address is often automatically generated from the interface MAC address) An IPv6 address is represented by 8 groups of 16-bit hexadecimal values separated by colons (:) shown as follows:
A typical example of an IPv6 address is
- 2001:0db8:85a3:0000:0000:8a2e:0370:7334
The hexadecimal digits are case-insensitive.
The 128-bit IPv6 address can be abbreviated with the following rules:
- Rule one: Leading zeroes within a 16-bit value may be omitted. For example, the address
fe80:0000:0000:0000:0202:b3ff:fe1e:8329may be written asfe80:0:0:0:202:b3ff:fe1e:8329 - Rule two: A single occurrence of consecutive groups of zeroes within an address may be replaced by a double colon. For example,
fe80:0:0:0:202:b3ff:fe1e:8329becomesfe80::202:b3ff:fe1e:8329
Dual IP stack implementation
The dual-stack protocol implementation in an operating system is a fundamental IPv4-to-IPv6 transition technology. It implements IPv4 and IPv6 protocol stacks either independently or in a hybrid form. The hybrid form is commonly implemented in modern operating systems supporting IPv6. Dual-stack hosts are described in RFC 4213.
Modern hybrid dual-stack implementations of IPv4 and IPv6 allow programmers to write networking code that works transparently on IPv4 or IPv6. The software may use hybrid sockets designed to accept both IPv4 and IPv6 packets. When used in IPv4 communications, hybrid stacks use an IPv6 application programming interface and represent IPv4 addresses in a special address format, the IPv4-mapped IPv6 address.
Proxying and translation for IPv6-only hosts
After the regional Internet registries have exhausted their pools of available IPv4 addresses, it is likely that hosts newly added to the Internet might only have IPv6 connectivity. For these clients to have backward-compatible connectivity to existing IPv4-only resources, suitable IPv6 transition mechanisms must be deployed.
One form of address translation is the use of a dual-stack application layer proxy server, for example a web proxy.
NAT-like techniques for application-agnostic translation at the lower layers in routers and gateways have been proposed. The NAT-PT standard was dropped due to a number of criticisms however more recently the continued low adoption of IPv6 has prompted a new standardization effort under the name NAT64.
IPv6 in the Domain Name System
In the Domain Name System, hostnames are mapped to IPv6 addresses by AAAA resource records, so-called quad-A records. For reverse resolution, the IETF reserved the domain ip6.arpa, where the name space is hierarchically divided by the 1-digit hexadecimal representation of nibble units (4 bits) of the IPv6 address. This scheme is defined in RFC 3596.
IPv4-mapped IPv6 addresses
Hybrid dual-stack IPv6/IPv4 implementations support a special class of addresses, the IPv4-mapped IPv6 addresses. This address type has its first 80 bits set to zero and the next 16 set to one, while its last 32 bits are filled with the IPv4 address. These addresses are commonly represented in the standard IPv6 format, but having the last 32 bits written in the customary dot-decimal notation of IPv4; for example, ::ffff:192.0.2.128 represents the IPv4 address 192.0.2.128. It substitutes the old and deprecated IPv4-compatible IPv6 address formed by ::192.0.2.128.
Because of the significant internal differences between IPv4 and IPv6, some of the lower level functionality available to programmers in the IPv6 stack do not work identically with IPv4 mapped addresses. Some common IPv6 stacks do not support the IPv4-mapped address feature, either because the IPv6 and IPv4 stacks are separate implementations (e.g., Microsoft Windows 2000, XP, and Server 2003), or because of security concerns (OpenBSD) On these operating systems, it is necessary to open a separate socket for each IP protocol that is to be supported. On some systems, e.g., the Linux kernel, NetBSD, and FreeBSD, this feature is controlled by the socket option IPV6_V6ONLY as specified in RFC 3493
Tunneling
In order to reach the IPv6 Internet, an isolated host or network must use the existing IPv4 infrastructure to carry IPv6 packets. This is done using a technique known as tunneling which consists of encapsulating IPv6 packets within IPv4, in effect using IPv4 as a link layer for IPv6.
The direct encapsulation of IPv6 datagrams within IPv4 packets is indicated by IP protocol number 41. IPv6 can also be encapsulated within UDP packets e.g. in order to cross a router or NAT device that blocks protocol 41 traffic. Other encapsulation schemes, such as used in AYIYA or GRE, are also popular.
Conversely, on IPv6-only internet links, when access to IPv4 network facilities are needed, tunneling of IPv4 over IPv6 protocol occurs, using the IPv6 as a link layer for IPv4.
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