SCIENCE DIRECT

IPv4 uses 32-bit (4-byte) addresses, which limits the address space to 4,294,967,296 (232) possible unique addresses. IPv4 reserves some addresses for special purposes such as private networks (~18 million addresses) or multicast addresses (~270 million addresses). This reduces the number of addresses that can be allocated to end users and, as the number of addresses available is consumed, IPv4 address exhaustion is inevitable. This foreseeable shortage was the primary motivation for developing IPv6, which is in various deployment stages around the world and is the only strategy for IPv4 replacement and continued Internet expansion.

IPv4 addresses are usually represented in dot-decimal notation (four numbers, each ranging from 0 to 255, separated by dots, e.g. 208.77.188.166). Each part represents 8 bits of the address, and is therefore called an octet. In less common cases of technical writing, IPv4 addresses may be presented in hexadecimal, octal, or binary representations. When converting, each octet is usually treated as a separate number.

In the early stages of development of the Internet protocol,network administrators interpreted an IP address as a structure of network number and host number. The highest order octet (most significant eight bits) was designated the network number and the rest of the bits were called the rest field or host identifier and were used for host numbering within a network. This method soon proved inadequate as additional networks developed that were independent from the existing networks already designated by a network number. In 1981, the Internet addressing specification was revised with the introduction of classful network architecture.

Classful network design allowed for a larger number of individual network assignments. The first three bits of the most significant octet of an IP address was defined as the class of the address. Three classes (A, B, and C) were defined for universal unicast addressing. Depending on the class derived, the network identification was based on octet boundary segments of the entire address. Each class used successively additional octets in the network identifier, thus reducing the possible number of hosts in the higher order classes (B and C).

Although classful network design was a successful developmental stage, it proved unscalable in the rapid expansion of the Internet and was abandoned when Classless Inter-Domain Routing (CIDR) was created for the allocation of IP address blocks and new rules of routing protocol packets using IPv4 addresses. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing on arbitrary-length prefixes.Today, remnants of classful network concepts function only in a limited scope as the default configuration parameters of some network software and hardware components (e.g. netmask), and in the technical jargon used in network administrators' discussions.

Early network design, when global end-to-end connectivity was envisioned for communications with all Internet hosts, intended that IP addresses be uniquely assigned to a particular computer or device. However, it was found that this was not always necessary as private networks developed and public address space needed to be conserved (IPv4 address exhaustion).

Computers not connected to the Internet, such as factory machines that communicate only with each other via TCP/IP, need not have globally-unique IP addresses. Three ranges of IPv4 addresses for private networks, one range for each class (A, B, C), were reserved in RFC 1918. These addresses are not routed on the Internet and thus their use need not be coordinated with an IP address registry.

Today, when needed, such private networks typically connect to the Internet through network address translation (NAT).

The rapid exhaustion of IPv4 address space, despite conservation techniques, prompted the Internet Engineering Task Force (IETF) to explore new technologies to expand the Internet's addressing capability. The permanent solution was deemed to be a redesign of the Internet Protocol itself. This next generation of the Internet Protocol, aimed to replace IPv4 on the Internet, was eventually named Internet Protocol Version 6 (IPv6) in 1995[3][4] The address size was increased from 32 to 128 bits or 16 octets, which, even with a generous assignment of network blocks, is deemed sufficient for the foreseeable future. Mathematically, the new address space provides the potential for a maximum of 2128, or about 3.403 × 1038 unique addresses.

The new design is not based on the goal to provide a sufficient quantity of addresses alone, but rather to allow efficient aggregation of subnet routing prefixes to occur at routing nodes. As a result, routing table sizes are smaller, and the smallest possible individual allocation is a subnet for 264 hosts, which is the size of the square of the size of the entire IPv4 Internet. At these levels, actual address utilization rates will be small on any IPv6 network segment. The new design also provides the opportunity to separate the addressing infrastructure of a network segment--that is the local administration of the segment's available space--from the addressing prefix used to route external traffic for a network. IPv6 has facilities that automatically change the routing prefix of entire networks should the global connectivity or the routing policy change without requiring internal redesign or renumbering.

he large number of IPv6 addresses allows large blocks to be assigned for specific purposes and, where appropriate, to be aggregated for efficient routing. With a large address space, there is not the need to have complex address conservation methods as used in classless inter-domain routing (CIDR).All modern[update] desktop and enterprise server operating systems include native support for the IPv6 protocol, but it is not yet widely deployed in other devices, such as home networking routers, voice over Internet Protocol (VoIP) and multimedia equipment, and network peripherals.

Just as IPv4 reserves addresses for private or internal networks, there are blocks of addresses set aside in IPv6 for private addresses. In IPv6, these are referred to as unique local addresses (ULA). RFC 4193 sets aside the routing prefix fc00::/7 for this block which is divided into two /8 blocks with different implied policies (cf. IPv6) The addresses include a 40-bit pseudorandom number that minimizes the risk of address collisions if sites merge or packets are misrouted.Early designs (RFC 3513) used a different block for this purpose (fec0::), dubbed site-local addresses. However, the definition of what constituted sites remained unclear and the poorly defined addressing policy created ambiguities for routing. The address range specification was abandoned and must no longer be used in new systems.

Addresses starting with fe80: — called link-local addresses — are assigned only in the local link area. The addresses are generated usually automatically by the operating system's IP layer for each network interface. This provides instant automatic network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, they have an instant communication path via their link-local IPv6 address. This feature is used extensively, and invisibly to most users, in the lower layers of IPv6 network administration (cf. Neighbor Discovery Protocol).

None of the private address prefixes may be routed in the public Internet.