Features and Differences From IPv4

To a great extent, IPv6 is a conservative extension of IPv4. Most transport- and application-layer protocols need little or no change to work over IPv6; exceptions are applications protocols that embed network-layer addresses (such as FTP or NTPv3).

IPv6 specifies a new packet format, designed to minimize packet-header processing. Since the headers of IPv4 and IPv6 are significantly different, the two protocols are not interoperable.

Larger address space

IPv6 features a larger address space than that of IPv4: addresses in IPv6 are 128 bits long versus 32 bits in IPv4.

The very large IPv6 address space supports 2128 (about 3.4×1038) addresses, or approximately 5×1028 (roughly 295) addresses for each of the roughly 6.5 billion (6.5×109) people alive today. In a different perspective, this is 252 addresses for every observable star in the known universe.

While these numbers are impressive, it was not the intent of the designers of the IPv6 address space to assure geographical saturation with usable addresses. Rather, the longer addresses allow a better, systematic, hierarchical allocation of addresses and efficient route aggregation. With IPv4, complex Classless Inter-Domain Routing (CIDR) techniques were developed to make the best use of the small address space. Renumbering an existing network for a new connectivity provider with different routing prefixes is a major effort with IPv4, as discussed in RFC 2071 and RFC 2072. With IPv6, however, changing the prefix in a few routers can renumber an entire network ad hoc, because the host identifiers (the least-significant 64 bits of an address) are decoupled from the subnet identifiers and the network provider's routing prefix.

The size of a subnet in IPv6 is 264 addresses (64-bit subnet mask); the square of the size of the entire IPv4 Internet. Thus, actual address space utilization rates will likely be small in IPv6, but network management and routing will be more efficient.

Stateless address autoconfiguration

IPv6 hosts can configure themselves automatically when connected to a routed IPv6 network using ICMPv6 router discovery messages. When first connected to a network, a host sends a link-local multicast router solicitation request for its configuration parameters; if configured suitably, routers respond to such a request with a router advertisement packet that contains network-layer configuration parameters.

If IPv6 stateless address autoconfiguration (SLAAC) proves unsuitable, a host can use stateful configuration (DHCPv6) or be configured manually. In particular, stateless autoconfiguration is not used by routers, these must be configured manually or by other means.

Multicast

Multicast, the ability to send a single packet to multiple destinations, is part of the base specification in IPv6. This is unlike IPv4, where it is optional (although usually implemented).

IPv6 does not implement broadcast, the ability to send a packet to all hosts on the attached link. The same effect can be achieved by sending a packet to the link-local all hosts multicast group.

Most environments, however, do not currently have their network infrastructures configured to route multicast packets; multicasting on single subnet will work, but global multicasting might not.

Mandatory network layer security

Internet Protocol Security (IPsec), the protocol for IP encryption and authentication, forms an integral part of the base protocol suite in IPv6. IPSec support is mandatory in IPv6; this is unlike IPv4, where it is optional (but usually implemented). IPsec, however, is not widely used at present except for securing traffic between IPv6 Border Gateway Protocol routers.

Simplified processing by routers

A number of simplifications have been made to the packet header, and the process of packet forwarding has been simplified, in order to make packet processing by routers simpler and hence more efficient. Concretely,

• The packet header in IPv6 is simpler than that used in IPv4, with many rarely-used fields moved to separate options; in effect, although the addresses in IPv6 are four times larger, the (option-less) IPv6 header is only twice the size of the (option-less) IPv4 header.
• IPv6 routers do not perform fragmentation. IPv6 hosts are required to either perform PMTU discovery, perform end-to-end fragmentation, or to send packets smaller than the IPv6 minimum maximum packet size.
• The IPv6 header is not protected by a checksum, integrity protection is expected to be assured by a transport-layer checksum. In effect, IPv6 routers do not need to recompute a checksum when header fields (such as the TTL or Hop Count) change. This improvement may have been made obsolete by the development of routers that perform checksum computation at line speed using dedicated hardware.
• The Time-to-Live field of IPv4 has been renamed to Hop Limit, reflecting the fact that routers are no longer expected to compute the time a packet has spent in a queue.

Mobility

Unlike mobile IPv4, Mobile IPv6 (MIPv6) avoids triangular routing and is therefore as efficient as normal IPv6. However, since neither MIPv6 nor MIPv4 are widely deployed today, this advantage is mostly theoretical.

Options Extensibility

IPv4 has a fixed size (40 bytes) of option parameters. In IPv6, options are implemented as additional extension headers after the IPv6 header, which limits their size only by the size of an entire packet.

Jumbograms

IPv4 limits packets to 64 KiB of payload. IPv6 has optional support for packets over this limit, referred to as jumbograms, which can be as large as 4 GiB. The use of jumbograms may improve performance over high-MTU networks. The presence of jumbograms is indicated by the Jumbo Payload Option header.

Source:
http://en.wikipedia.org/wiki/IPv6