Characterization of Features in IPv6
IPv6 retains many of the design features that have made IPv4 so successful. Like IPv4, IPv6 is connectionless; each datagram is routed independently. Like IPv4, the header in a datagram contains a maximum number of hops the datagram can take before being discarded. More important, IPv6 retains most of the general facilities provided by the IPv4 options.
Despite retaining the basic concepts from the current version, IPv6 has changes in all the details. For example, IPv6 uses large addresses and an entirely new datagram header format. Finally, IPv6 uses a series of fixed-length header to handle optional information instead of a single header with a variable-length options field.

The new features in IPv6 may be grouped into five main categories:
(1) Address size: Instead of 32 bits, each IPv6 address contains 128 bits. The resulting address space is large enough to accommodate continued growth of the World-Wide Internet for many decades.
(2) Header format:    The IPv6 datagram header is completely different from the IPv4 header. Almost every field in the header has been changed; some have been replaced.
(3) Extension header: Unlike IPv4, which uses a single header format for all datagrarns, IPv6 encodes information into separate headers. A datagram consists of the base IPv6 header followed by zero, or more extension headers followed by data.
(4) Support for audio and video: IPv6 includes a mechanism that allows a sender and receiver to establish a high-quality path through the underlying network and to associate datagrams with that path. Although the mechanism is intended for use with audio and video applications that require high performance guarantee, the mechanism can also be used to associate datagrams with low-cost paths.
(5) Extensible protocol:    Unlike IPv4, IPv6 does not specify all possible protocol features. Instead, the designers have provided a scheme that allows a sender to add additional information to a datagram. The extension scheme makes IPv6 more flexible than IPv4 and means that new features can be added to the design as needed.
To distinguish the new version of IP from the current version, the two protocols are referred to using their version numbers. The current version of IP is IPv4 and the proposed new version is IPv6.
Though IPv6 retains many of the concepts of IPv4, it differs from the latter in all the details. For example, like IPv4, IPv6 provides a connectionless service in which two computers exchange short messages called datagrams. However, unlike an IPv4 datagram in which the header contains fields for each function, IPv6 defines separate headers for each function, Each IPv6 datagram consists of a base header followed by zero or more extension header followed by data.
Like IPv4, IPv6 defines an address for each network connection. Thus, as in IPv4, a computer that connects to multiple physical networks (say, a router) has multiple addresses. However, special addresses are completely changed in IM. Instead of IPv4's notation of network broadcast, IPv6 defines multicast and any cast (cluster) addresses, both of which correspond to a set of computers. A multicast address corresponds to a set of computers at multiple sites that are treated as a single entity; each computer in the set will receive a copy of any datagram sent to the set. A cluster permits replication of services: a datagram sent to a cluster address will be delivered to exactly one member of the cluster (for example, the member that is closest to the sender).

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