IPv6 is the new version of Internet Protocol (IP) based on IPv4, a network-layer (Layer 3) protocol that contains addressing information and some control information enabling packets to be routed in the network. There are two basic IP versions: IPv4 and IPv6. IPv6 is also called next generation IP or IPng. IPv4 and IPv6 are de-multiplexed at the media layer. For example, IPv6 packets are carried over Ethernet with the content type 86DD (hexadecimal) instead of IPv4's 0800. The IPv4 is described in separate documents. This document describes the IPv6 details.

IPv6 increases the IP address size from 32 bits to 128 bits, to support more levels of addressing hierarchy, a much greater number of addressable nodes, and simpler auto-configuration of addresses. IPv6 addresses are expressed in hexadecimal format (base 16) which allows not only numerals (0-9) but a few characters as well (a-f). A sample ipv6 address looks like: 3ffe:ffff:100:f101:210:a4ff:fee3:9566. Scalability of multicast addresses is introduced. A new type of address called an anycast address is also defined, to send a packet to any one of a group of nodes. Two major improvements in IPv6 vs. v4:

• Improved support for extensions and options - IPv6 options are placed in separate headers that are located between the IPv6 header and the transport layer header. Changes in the way IP header options are encoded to allow more efficient forwarding, less stringent limits on the length of options, and greater flexibility for introducing new options in the future.
• Flow labeling capability - A new capability has been added to enable the labeling of packets belonging to particular traffic flows for which the sender requests special handling, such as non-default Quality of Service or real-time service.

Protocol Structure

• Version - Internet Protocol Version number (For IPv6, it is 6).
• Priority -- Traffic class field enables a source to identify the desired delivery priority of the packets. Priority values are divided into ranges: traffic where the source provides congestion control and non-congestion control traffic.
• Flow label -- Flow label is used by a source to label those products for which it requests special handling by the IPv6 router. The flow is uniquely identified by the combination of a source address and a non-zero flow label.
• Payload length -- The length of the data portion of the packet.
• Next header -- Identifies the type of header immediately following the IPv6 header.
• Hop limit -- It is decremented by one by each node that forwards the packet. The packet is discarded if the Hop Limit is decremented to zero.
• Source address -- 128-bit address of the originator of the packet .
• Destination address -- 128-bit address of the intended recipient of the packet (possibly not the ultimate recipient, if a Routing header is present).

Related Terms: IP, IPv4, TCP, UDP, ICMP, SNMP, FTP, TELNET, SMTP, ARP, RARP, RPC, XDR, and NFS

Sponsor Source: IPv6 is defined by IETF (http://www.ietf.org) RFC 1883 (original) and RFC 2460 (latest)

Reference:
http://www.javvin.com/protocol/rfc1883.pdf: IPv6 Specifications (original)
http://www.javvin.com/protocol/rfc2460.pdf: IPv6 specifications (the latest)
http://www.ipv6forum.com: A good informational site for IPv6