Most of the changes from IPv4 to IPv6 are described in the following categories:
Expanded routing and addressing capabilities – IPv6 increases the IP address size from 32 bits to 128 bits to support more levels of addressing hierarchy. IPv6 provides a much greater number of addressable nodes and employs simpler autoconfiguration of addresses.
The addition of a scope field improves the scalability of multicast routing to multicast addresses.
IPv6 defines a new type of address that is called an anycast address. An anycast address identifies sets of nodes. A packet that is sent to an anycast address is delivered to one of the nodes. The use of anycast addresses in the IPv6 source route allows nodes to control the path over which their traffic flows.
Header format simplification – Some IPv4 header fields have been dropped or have been made optional. This change reduces the common-case processing cost of packet handling. This change also keeps the bandwidth cost of the IPv6 header as low as possible, despite the increased size of the addresses. Even though the IPv6 addresses are four times longer than the IPv4 addresses, the IPv6 header is only twice the size of the IPv4 header.
Improved support for options – Changes in the way IP header options are encoded allow for more efficient forwarding. Also, the length of options has less stringent limits. The changes also provide greater flexibility for introducing new options in the future.
Quality-of-service capabilities – A new capability is added to enable the labeling of packets that belong to particular traffic flows for which the sender requests special handling. For example, the sender can request nondefault quality of service or real-time service.
Authentication and privacy capabilities – IPv6 includes the definition of extensions that provide support for authentication, data integrity, and confidentiality.