The Internet operates by transferring data between hosts in packets that are routed across networks as specified by routing protocols. These packets require an addressing scheme, such as IPv4 or IPv6, to specify their source and destination addresses. Each host, computer or other device on the Internet requires an IP address in order to communicate. The growth of the Internet has created a need for more addresses than are possible with IPv4. The last top level (
[[CIDR|/8]]) block of free IPv4 addresses was assigned in February 2011 by IANA to the 5 RIRs, although many free addresses still remain in most assigned blocks and RIRs will continue with standard policy until it is at its last
/8. After that, only 1024 addresses (a
/22) are made available from the RIR for each LIR: currently, only APNIC has already reached this stage.
IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with this long-anticipated IPv4 address exhaustion, and is described in Internet standard document RFC 2460, published in December 1998. Like IPv4, IPv6 is an internet-layer protocol for packet-switched internetworking and provides end-to-end datagram transmission across multiple IP networks. While IPv4 allows 32 bits for an IP address, and therefore has 232 possible addresses, IPv6 uses 128-bit addresses, for an address space of 2128 (approximately) addresses. This expansion allows for many more devices and users on the internet as well as extra flexibility in allocating addresses and efficiency for routing traffic. It also eliminates the primary need for network address translation (NAT), which gained widespread deployment as an effort to alleviate IPv4 address exhaustion.
IPv6 also implements additional features not present in IPv4. It simplifies aspects of address assignment (stateless address autoconfiguration), network renumbering and router announcements when changing Internet connectivity providers. The IPv6 subnet size has been standardized by fixing the size of the host identifier portion of an address to 64 bits to facilitate an automatic mechanism for forming the host identifier from link-layer media addressing information (MAC address). Network security is also integrated into the design of the IPv6 architecture, including the option of IPsec.
For the Internet to make use of the advantages of IPv6 over IPv4, most hosts on the Internet, as well as the networks connecting them, will need to deploy this protocol. However, IPv6 deployment is difficult. While deployment of IPv6 is accelerating, especially in the Asia-Pacific region and some European countries, areas such as the Americas and Africa are comparatively lagging in deployment of IPv6. IPv6 does not implement interoperability features with IPv4, and creates essentially a parallel, independent network. Exchanging traffic between the two networks requires special translator gateways, but modern computer operating systems implement dual-protocol software for transparent access to both networks either natively or using a tunneling protocol such as 6to4, 6in4, or Teredo. In December 2010, despite marking its 12th anniversary as a Standards Track protocol, IPv6 was only in its infancy in terms of general worldwide deployment. A 2008 study by Google Inc indicated that penetration was still less than one percent of Internet-enabled hosts in any country at that time.
See main article: IPv4.
The first publicly used version of the Internet Protocol Version 4 (IPv4), provides an addressing capability of 232 or approximately 4.3 billion addresses. Address exhaustion was not initially a concern in IPv4 as this version was originally presumed to be an internal test within ARPA, and not intended for public use.
During the first decade of operation of the Internet (by the late 1980s), it became apparent that methods had to be developed to conserve address space. In the early 1990s, even after the redesign of the addressing system using a classless network model, it became clear that this would not suffice to prevent IPv4 address exhaustion, and that further changes to the Internet infrastructure were needed.
By the beginning of 1992, several proposals appeared and by the end of 1992, the IETF announced a call for white papers. In September 1993, the IETF created a temporary, ad-hoc IP Next Generation (IPng) area to deal specifically with IPng issues. The new area was led by Allison Mankin and Scott Bradner, and had a directorate with 15 engineers from diverse backgrounds for direction-setting and preliminary document review:  The working-group members were J. Allard (Microsoft), Steve Bellovin (AT&T), Jim Bound (Digital Equipment Corporation), Ross Callon (Wellfleet), Brian Carpenter (CERN), Dave Clark (MIT), John Curran (NEARNET), Steve Deering (Xerox), Dino Farinacci (Cisco), Paul Francis (NTT), Eric Fleischmann (Boeing), Mark Knopper (Ameritech), Greg Minshall (Novell), Rob Ullmann (Lotus), and Lixia Zhang (Xerox).
The Internet Engineering Task Force adopted the IPng model on July 25, 1994, with the formation of several IPng working groups. By 1996, a series of RFCs was released defining Internet Protocol version 6 (IPv6), starting with RFC 1883. (Version 5 was used by the experimental Internet Stream Protocol.)
It is widely expected that the Internet will use IPv4 alongside IPv6 for the foreseeable future. IPv4-only and IPv6-only nodes cannot communicate directly, and need assistance from an intermediary gateway or must use other transition mechanisms.
See main article: IPv4 address exhaustion.
On February 3, 2011, in a ceremony in Miami, the Internet Assigned Numbers Authority (IANA) assigned the last batch of 5
/8 address blocks to the Regional Internet Registries, officially depleting the global pool of completely fresh blocks of addresses. Each of the address blocks represents approximately 16.7 million possible addresses, or over 80 million combined potential addresses.
These addresses could well be fully consumed within three to six months of that time at current rates of allocation. APNIC was the first RIR to exhaust its regional pool on 15 April 2011, except for a small amount of address space reserved for the transition to IPv6, which will be allocated in a much more restricted way.
In 2003, the director of Asia-Pacific Network Information Centre (APNIC), Paul Wilson, stated that, based on then-current rates of deployment, the available space would last for one or two decades. In September 2005, a report by Cisco Systems suggested that the pool of available addresses would exhaust in as little as 4 to 5 years. In 2008, a policy process started for the end-game and post-exhaustion era. In 2010, a daily updated report projected the global address pool exhaustion by the first quarter of 2011, and depletion at the five regional Internet registries before the end of 2011.
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.
thumb|Decomposition of an IPv6 address into its binary form
The main advantage of IPv6 over IPv4 is its larger address space. The length of an IPv6 address is 128 bits, compared to 32 bits in IPv4. The address space therefore has 2128 or approximately addresses. By comparison, this amounts to approximately addresses for each of the seven billion people alive in 2011. In addition, the IPv4 address space is poorly allocated, with approximately 14% of all available addresses utilized. While these numbers are 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.
Multicasting, the transmission of a packet to multiple destinations in a single send operation, is part of the base specification in IPv6. In IPv4 this is an optional although commonly implemented feature. IPv6 multicast addressing shares common features and protocols with IPv4 multicast, but also provides changes and improvements by eliminating the need for certain protocols. IPv6 does not implement traditional IP broadcast, i.e. the transmission of a packet to all hosts on the attached link using a special broadcast address, and therefore does not define broadcast addresses. In IPv6, the same result can be achieved by sending a packet to the link-local all nodes multicast group at address
ff02::1, which is analogous to IPv4 multicast to address
184.108.40.206. IPv6 also provides for new multicast implementations, including embedding rendezvous point addresses in an IPv6 multicast group address, which simplifies the deployment of inter-domain solutions.
In IPv4 it is very difficult for an organization to get even one globally routable multicast group assignment, and the implementation of inter-domain solutions is very arcane. Unicast address assignments by a local Internet registry for IPv6 have at least a 64-bit routing prefix, yielding the smallest subnet size available in IPv6 (also 64 bits). With such an assignment it is possible to embed the unicast address prefix into the IPv6 multicast address format, while still providing a 32-bit block, the least significant bits of the address, or approximately 4.2 billion multicast group identifiers. Thus each user of an IPv6 subnet automatically has available a set of globally routable source-specific multicast groups for multicast applications.
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.
Internet Protocol Security (IPsec) was originally developed for IPv6, but found widespread deployment first in IPv4, into which it was back-engineered. Earlier, IPsec was an integral part of the base IPv6 protocol suite,  but has since been made optional.
In IPv6, the packet header and the process of packet forwarding have been simplified. Although IPv6 packet headers are at least twice the size of IPv4 packet headers, packet processing by routers is generally more efficient,  thereby extending the end-to-end principle of Internet design. Specifically:
Unlike mobile IPv4, mobile IPv6 avoids triangular routing and is therefore as efficient as native IPv6. IPv6 routers may also allow entire subnets to move to a new router connection point without renumbering.
The IPv6 protocol header has a fixed size (40 octets). Options are implemented as additional extension headers after the IPv6 header, which limits their size only by the size of an entire packet. The extension header mechanism makes the protocol extensible in that it allows future services for quality of service, security, mobility, and others to be added without redesign of the basic protocol.
IPv4 limits packets to (216−1) octets of payload. An IPv6 node can optionally handle packets over this limit, referred to as jumbograms, which can be as large as (232−1) octets. The use of jumbograms may improve performance over high-MTU links. The use of jumbograms is indicated by the Jumbo Payload Option header.
Like IPv4, IPv6 supports globally unique static IP addresses, which can be used to track a single device's Internet activity. Most devices are used by a single user, so a device's activity is often assumed to be equivalent to a user's activity. This is a cause for concern to anyone who has political, social, or economic reasons for keeping their Internet activity secret.
Activity tracking based on IP address is a potential privacy issue for all IP-enabled devices. However, device activity can be particularly simple to track when the host identifier portion of the IPv6 address is automatically generated from the network interface's MAC address.
Privacy extensions for IPv6 have been defined to address these privacy concerns. When privacy extensions are enabled, the operating system generates ephemeral IP addresses by concatenating a randomly generated host identifier with the assigned network prefix. These ephemeral addresses, instead of trackable static IP addresses, are used to communicate with remote hosts. The use of ephemeral addresses makes it difficult to accurately track a user's Internet activity by scanning activity streams for a single IPv6 address.
Privacy extensions do not protect the user from other forms of activity tracking, such as tracking cookies. Privacy extensions do little to protect the user from tracking if only one or two hosts are using a given network prefix, and the activity tracker is privy to this information. In this scenario, the network prefix is the unique identifier for tracking. Network prefix tracking is less of a concern if the user's ISP assigns a dynamic network prefix via DHCP. 
See main article: IPv6 packet.
An IPv6 packet has two parts: a header and payload.
The header consists of a fixed portion with minimal functionality required for all packets and may contain optional extensions to implement special features.
The fixed header occupies the first 40 octets (320 bits) of the IPv6 packet. It contains the source and destination addresses, traffic classification options, a hop counter, and a pointer for extension headers, if any. The Next Header field, present in each extension, points to the next element in the chain of extensions. The last field points to the upper-layer protocol that is carried in the packet's payload.
Extension headers carry options that are used for special treatment of a packet in the network, e.g., for routing, fragmentation, and for security using the IPsec framework.
Without special options, a payload must be less than . With a Jumbo Payload option (in a Hop-By-Hop Options extension header), the payload must be less than 4 GB.
Unlike in IPv4, routers never fragment a packet. Hosts are expected to use Path MTU Discovery to make their packets small enough to reach the destination without needing to be fragmented. See IPv6 Packet#Fragmentation.
See main article: IPv6 address.
Compared to IPv4, the most obvious advantage of IPv6 is its larger address space. IPv4 addresses are 32 bits long and number about (4.3 billion). IPv6 addresses are 128 bits long and number about . IPv6's addresses are deemed enough for the foreseeable future.
IPv6 addresses are written in eight groups of four hexadecimal digits separated by colons, such as
2001:0db8:85a3:0000:0000:8a2e:0370:7334. IPv6 unicast addresses other than those that start with binary 000 are logically divided into two parts: a 64-bit (sub-)network prefix, and a 64-bit interface identifier.
For stateless address autoconfiguration (SLAAC) to work, subnets require a /64 address block, as defined in RFC 4291 section 2.5.1. 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. It remains to be seen if ISPs will honor this recommendation; for example, during initial trials, Comcast customers were given a single /64 network.
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 loopback, 6to4 tunneling, and Teredo tunneling. See RFC 5156. Also, some address ranges are considered special, such as link-local addresses for use on the local link only, Unique Local addresses (ULA) as described in RFC 4193, and solicited-node multicast addresses used in the Neighbor Discovery Protocol.
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
[[.arpa|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.
An IPv6 address is represented by 8 groups of 16-bit hexadecimal values separated by colons (:). For example:
The hexadecimal digits are case-insensitive.
An IPv6 address can be abbreviated with the following rules:
Below is an example of these rules:
|After Rule 1||fe80||0||0||0||202||b3ff||fe1e||8329|
|After Rule 2||fe80||202||b3ff||fe1e||8329|
Below are the text representations of these addresses:
An IPv6 address may have more than one representation, but RFC 5952 recommends a canonical text representation.
Until IPv6 completely supplants IPv4, a number of transition mechanisms are needed to enable IPv6-only hosts to reach IPv4 services and to allow isolated IPv6 hosts and networks to reach the IPv6 Internet over the IPv4 infrastructure. People have made various proposals for this transition period:
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 that implement 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.
Hybrid dual-stack IPv6/IPv4 implementations recognize a special class of addresses, the IPv4-mapped IPv6 addresses. In these addresses, the first 80 bits are zero, the next 16 bits are one, and the remaining 32 bits are the IPv4 address. You may see these addresses with the first 96 bits written in the standard IPv6 format, and the remaining 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. A deprecated format for IPv4-compatible IPv6 addresses was
Because of the significant internal differences between IPv4 and IPv6, some of the lower-level functionality available to programmers in the IPv6 stack does not work identically with IPv4-mapped addresses. Some common IPv6 stacks do not implement 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, a program must open a separate socket for each IP protocol it uses. 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.
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 encapsulates IPv6 packets within IPv4, in effect using IPv4 as a link layer for IPv6.
IP protocol 41 indicates IPv4 packets which encapsulate IPv6 datagrams. Some routers or network address translation devices may block protocol 41. To pass through these devices, you might use UDP packets to encapsulate IPv6 datagrams. Other encapsulation schemes, such as AYIYA or Generic Routing Encapsulation, are also popular.
Conversely, on IPv6-only internet links, when access to IPv4 network facilities is needed, tunneling of IPv4 over IPv6 protocol occurs, using the IPv6 as a link layer for IPv4.
Automatic tunneling refers to a technique where the routing infrastructure automatically determines the tunnel endpoints. Some automatic tunneling techniques are below.
6to4 is recommended by RFC 3056. It uses protocol 41 encapsulation. Tunnel endpoints are determined by using a well-known IPv4 anycast address on the remote side, and embedding IPv4 address information within IPv6 addresses on the local side. 6to4 is widely deployed today.
Teredo is an automatic tunneling technique that uses UDP encapsulation and can allegedly cross multiple NAT boxes. IPv6, including 6to4 and Teredo tunneling, are enabled by default in Windows Vista and Windows 7. Most Unix systems implement only 6to4, but Teredo can be provided by third-party software such as Miredo.
ISATAP treats the IPv4 network as a virtual IPv6 local link, with mappings from each IPv4 address to a link-local IPv6 address. Unlike 6to4 and Teredo, which are inter-site tunnelling mechanisms, ISATAP is an intra-site mechanism, meaning that it is designed to provide IPv6 connectivity between nodes within a single organisation.
In configured tunneling, the tunnel endpoints are explicitly configured, either by an administrator manually or the operating system's configuration mechanisms, or by an automatic service known as a tunnel broker; this is also referred to as automated tunneling. Configured tunneling is usually more deterministic and easier to debug than automatic tunneling, and is therefore recommended for large, well-administered networks. Automated tunneling provides a compromise between the ease of use of automatic tunneling and the deterministic behaviour of configured tunneling.
Raw encapsulation of IPv6 packets using IPv4 protocol number 41 is recommended for configured tunneling; this is sometimes known as 6in4 tunneling. As with automatic tunneling, encapsulation within UDP may be used in order to cross NAT boxes and firewalls.
See main article: IPv6 transition mechanisms.
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.
RFC 4038, Application Aspects of IPv6 Transition, is an informational RFC that covers the topic of IPv4 to IPv6 application transition mechanisms. Other RFCs that pertain IPv6 at the application level are:
Similar to the OS-level WAN stack, applications can be:
Compatibility with IPv6 networking is mainly a software or firmware issue. However, much of the older hardware that could in principle be upgraded is likely to be replaced instead. The American Registry for Internet Numbers (ARIN) suggests that all Internet servers be prepared to serve IPv6-only clients by January 2012.
Most personal computers running recent operating system versions are IPv6-ready. Most popular applications with network capabilities are ready, and most others could be easily upgraded with help from the developers. Java applications adhering to Java 1.4 (February 2002) standards work with IPv6.
Low-level equipment such as network adapters and network switches may not be affected by the change, since they transmit link-layer frames without inspecting the contents. However, networking devices that obtain IP addresses or perform routing of IP packets do need to understand IPv6.
Most equipment would be IPv6 capable with a software or firmware update if the device has sufficient storage and memory space for the new IPv6 stack. However, manufacturers may be reluctant to spend on software development costs for hardware they have already sold when they are poised for new sales from IPv6-ready equipment.
In some cases, non-compliant equipment needs to be replaced because the manufacturer no longer exists or software updates are not possible, for example, because the network stack is implemented in permanent read-only memory.
The CableLabs consortium published the 160 Mbit/s DOCSIS 3.0 IPv6-ready specification for cable modems in August 2006. The widely used DOCSIS 2.0 does not support IPv6. The new 'DOCSIS 2.0 + IPv6' standard supports IPv6, which may on the cable modem side require only a firmware upgrade.  It is expected that only 60% of cable modems' servers and 40% of cable modems will be DOCSIS 3.0 by 2011. However, most ISPs that support DOCSIS 3.0 do not support IPv6 across their networks.
Other equipment which are typically not IPv6-ready ranges from Voice over Internet Protocol devices to laboratory equipment and printers.
See main article: IPv6 deployment.
The introduction of Classless Inter-Domain Routing (CIDR) in the Internet routing and IP address allocation methods in 1993 and the extensive use of network address translation (NAT) delayed the inevitable IPv4 address exhaustion, but the final phase of exhaustion started on February 3, 2011. However, despite a decade long development and implementation history as a Standards Track protocol, general worldwide deployment is still in its infancy. As of October 2011, about 3% of domain names and 12% of the networks on the internet have IPv6 protocol support.
Nevertheless, IPv6 has been implemented on all major operating systems in use in commercial, business, and home consumer environments. Since 2008, the domain name system can be used in IPv6 as major web sites like Google, although sometimes with extra configuration. IPv6 was first used in a major world event during the 2008 Summer Olympic Games, the largest showcase of IPv6 technology since the inception of IPv6. Countries like China or the Federal U.S. Government are also starting to require IPv6 capability on their equipment.
Finally, modern cellular telephone specifications mandate IPv6 operation and deprecate IPv4 as an optional capability.