Every computer or other device that connects to the Internet requires a unique Internet protocol (IP) address. Internet protocol version 4 (IPv4) uses 32-bit addresses that allow 232 or about 4.3 billion unique IP addresses. The IP address space is managed by the Internet Assigned Numbers Authority (IANA) globally, and by five Regional Internet Registrie (RIRs) responsible in their designated territories for assignment to end users and local Internet registries, such as Internet service providers—according to Wikipedia.

Fig. 1: Addressing scheme in IPv6
Fig. 1: Addressing scheme in IPv6
Fig. 2: Structure of an IPv6 packet
Fig. 2: Structure of an IPv6 packet

In the late 1970s, when the IPv4 address space was designed, it was believed that its 4.29 billion addresses would be sufficient to cover any futur growth of the Internet. However, with advances in technology and an allocation practice that did not anticipate the explosion of hosts on the Internet, by 1992 it was clear that a replacement would be necessary.

IPv6: the saviour
In December 1998, the Internet Engineering Task Force (IETF) developed IPv6 to deal with exhaustion of IPv4 addresses, and described it in Request For Comments (RFC 2460). IPv6 uses an addressing scheme based on a unique Internet address of 128 bits, as opposed to 32 bits of IPv4. The 128-bit address space allows for 2128 or 3.4×1038 possible addresses, which seem quite adequate for the foreseeable future.

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Addressing. Use of 128 bits allows multiple levels of hierarchy and flexibility in designing hierarchical addressing and routing, which the current IPv4 Internet lacks. IPv6 addresses are represented in colon-hexadecimal format. The addresses are divided along 16-bit boundaries. The 16-bit blocks are converted into 4-digit hexadecimal numbers and separated by colons.

Fig. 3: IPv4 and IPv6 headers
Fig. 3: IPv4 and IPv6 headers

IPv6 addresses are classifiedinto several groups such as unicast (one-to-one), multicast (one-to-many) and anycast (one-to-nearest). IPv6 does not definea broadcast address and all types of IPv4 broadcast addresses are performed in IPv6 using multicast addresses.

Packet. An IPv6 packet contains IPv6 header, extension headers and upper-layer protocol data unit. The IPv6 header is always present and fixed at 40 bytes in size.

IPv4 and IPv6 interoperability and transition
According to Wikipedia, 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. A wide range of techniques have been identifiedand implemented by IETF for this purpose. The techniques can be broadly categorised as:

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1. Dual IP stack techniques that allow IPv4 and IPv6 to coexist in the same devices and networks
2. Tunneling techniques that connect IPv6 domains via IPv4 clouds
3. Translation techniques that provide an interconnection between IPv4 and IPv6 domains to allow IPv6-only devices to communicate with IPv4-only devices

Dual IP stack. The basic IPv4-to-IPv6 migration technology uses dual IP stack protocol architecture, which connects IPv4/IPv6 nodes to IPv4-only nodes or IPv6-only nodes. This implementation incorporates IPv4 and IPv6 protocol stacks either independently or in a hybrid form. The application can use either protocol.

Fig. 4: Dual-IP stack architecture
Fig. 4: Dual-IP stack architecture
Fig. 5: Tunneling using 6-to-4
Fig. 5: Tunneling using 6-to-4

Dual-stack implementation has its own transport layer, which includes transmission control protocol (TCP) and user datagram protocol (UDP), and framing layer, which performs link layer encapsulation and decapsulation. Present systems share common transport and framing layers and are enabled by default.

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. This address type has its first80 bits set to ‘0’ and the next 16 bits set to ‘1,’ while last 32 bits are filledwith the IPv4 address.

Tunneling. Tunneling is a technique in which an isolated IPv6 host can communicate via IPv4 infrastructure. This is achieved by encapsulating IPv6 packets within IPv4, which is used as a link layer for IPv6. IP protocol number 41 indicates encapsulation of IPv6 datagrams within IPv4 packets.

Translation and proxy for IPv6- only hosts. In order to provide backward-compatibility for IPv6-only hosts to existing IPv4-only resources, translation and proxy mechanisms may be used. Network address translator (NAT) like technique (NAT64) for translation at the lower layers in routers and gateways may be considered. Dual-stack application-layer proxy server, a Web proxy, can also be used for address translation.

Benefits of IPv6 over Ipv4
Larger IP address space. An enhanced address space, in the absence of NAT, provides less complex network hardware and software. Configuringa network becomes much simpler. Finally, the large availability of IP addresses removes any obstacle that existed previously in the full deployment of wireless and mobile devices.

Simplified and modular header structure. IPv6 has a simpler packet header structure than IPv4, designed to minimise the time and efforts that go into header processing. IPv6 headers are processed more efficiently at the intermediate routers, without having to parse through headers or recompute network-layer checksums, or even fragment and reassemble packets. Moreover, IPv6 extension headers can be expanded to accommodate what-ever extension data is thought necessary for efficient IPv6 communication.

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A number of transition mechanisms such as dual IPv4/IPv6 stack, tunneling, dual IP stack using network address and port translation, proxy and translation techniques have been defined that allow IPv4 and IPv6 to coexist till the time complete migration to IPv6 is not feasible

Better security for applications and networks. In IPv6, IPSec is a major protocol requirement and one of the factors in ensuring that IPv6 provides better security than IPv4. It contains a set of cryptographic protocols for ensuring secure data communication and key exchange. The main protocols used are authentication header protocol, encapsulating security payload protocol and Internet key exchange protocol.

End-to-end security mechanisms provide authentication and encryption abilities to all applications. Thus IPv6 eliminates the need for applications themselves to have integrated support for such abilities.

Better end-to-end connectivity. In the absence of unique IP addresses for each end, NAT creates difficultyin ensuring proper end-to-end services. IPv6 with its large address space no longer requires NAT and can ensure true end-to-end connectivity. This means peer-to-peer applications like VoIP and streaming media can work very effectively and efficientlywith Ipv6.

Better auto-configuration. IPv4 uses the stateful address auto-configuratio protocol—dynamic host configuratio protocol (DHCP)—to configure the node plugged into the network.

IPv6 too offers automatic configration but, more importantly, simple configurationmechanisms, known as plug-and-play auto-configuration. It o-fers DHCPv6, which is an auto-confi-uration protocol similar to IPv4 DHCP that offers stateful address auto-configuration.IPv6 also offers stateless or serverless address auto-configuration which enables a host to automatically configure its own IPv6 address.

Better QoS/prioritised delivery. Quality of Service (QoS) is given a special boost in the IPv6 protocol. The IPv6 header contains a new fiel called ‘flow label,’ which defines h packets are identifie and handled quickly and efficientlyby the routers. This ensures a high degree of QoS especially for peer-to-peer applications like VoIP and other real-time applications.

Better mobility features. With IPv6, mobility support using Mobile IPv6 (MIPv6) has been made mandatory. Route optimisation is a built-in feature for Mobile IPv6. Further, features like neighbour discovery and address auto-configurationallow mobile nodes to function in any location without requiring the services of any special router. MIPv6 can be used to achieve seamless mobility by allowing handovers between different access technologies. There is no ingress-fitering problem in Mobile IPv6 because the correspondent node uses care-of-address (address of present location) as the source address.

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Better multicast and anycast abilities. IPv6 extends the multicasting capabilities of IPv4 by offering a large multicast address range. Obviously, this limits the degree to which the information packets have to be propagated, and signifiantly improves the network efficiency.Multicasting requires the presence of fewer packets in the network, thereby optimising bandwidth utilisation and also reducing the resources required at each network node.

IPv6 also improves dramatically on the concept of anycast services, which are available in a very minimal form in IPv4.

Ease of administration. IPv6 provides automatic network renumbering, which enables smoother switchovers or mergers. Another useful administrative feature of IPv6 is its multihoming technique, using which simultaneous connections are established to two Internet service providers (ISPs). When service to one ISP is lost, a back-up connection to the Internet ensures greater reliability of services.

Efficiet, hierarchical addressing and routing infrastructure. IPv6 global addresses used on the IPv6 portion of the Internet are designed to create an efficient,hierarchical and summarisable routing infrastructure that is based on the common occurrence of multiple levels of Internet service providers.

Smooth transition from Ipv4
A number of transition mechanisms such as dual IPv4/IPv6 stack, tunneling, dual IP stack using network address and port translation, proxy and translation techniques have been definedthat allow IPv4 and IPv6 to coexist till the time complete migration to IPv6 is not feasible.

The IPv6 format will allow countless network addresses and almost eliminate the need for NAT. Nevertheless, the number of IP addresses should not be considered as the only reason to move to the new protocol. Other characteristics like greater network scalability, security, data integrity, QoS, auto configuration and bette network routing needed for new business needs have also been integrated in IPv6, which thus holds the promise of achieving end-to-end security, mobile communications, quality of service and simplified system management.

The author works at Bharat Sanchar Nigam Limited as junior telecom officer in Ludhiana, Punjab, and is pursuing PhD in electronics engineering from Indian Institute of Technology, Banaras Hindu University, Varanasi