IPv4 to IPv6 transition is required:

More Efficient Routing:

• IPv6 reduces the size of routing tables and makes routing more efficient.
• IPv6 lets ISPs aggregate the prefixes of their customers' networks into a single prefix and announce this one prefix to the IPv6 Internet.
• More Efficient Packet Processing: IPv6's simplified packet header makes packet processing way more effective.
• Compared to IPv4, IPv6 has no IP-level checksum, so it does not need to be recalculated at every hop of the router.

Directed Data Flows:

• IPv6 supports multicast more than broadcast.
• Multicast lets bandwidth-intensive flows (like multimedia streams) to be sent across to multiple destinations simultaneously, saving the network bandwidth to the greatest extent.
• The IPv6 header has a new field, named Flow Label that can identify packets belonging to the same flow.

Simplified Network Configuration:

• Address auto-configuration (address assignment) is built in to IPv6.
• A router will send the prefix of the local link in its router commercials.
• A host, is capable of generatng its independent IP address by appending its link-layer address, by converting the same into an Extended Universal Identifier (EUI) 64-bit format, to the 64 bits of the local link prefix.

Support for New Services:

• By eliminating Network Address Translation (NAT), true end-to-end connectivity at the IP layer is hence retained by enabling new and more valuable services.
• The advantage with Peer-to-peer network solutions is that they are way easier to develop and maintain, and services such as VoIP and Quality of Service (QoS) become more robust.

Security:

• IPsec, which provides privacy, authenticity and data compatibility, is wired into in IPv6.
• Due to its ability to carry forward malware, IPv4 ICMP packets are most often blocked by firewalls that are corporate centric, but ICMPv6, the execution of the Cyber Controlled Message Protocol for IPv6, could be allowed because IPSec can be implemented in the ICMPv6 packets as well.

Various techniques used in transition:

• The following transition techniques provide a mechanism to gradually migrate from an IPv4 infrastructure to an IPv6 infrastructure:
• Dual Stack - The ability to assign an IPv4 and an IPv6 address to the same network interface
• Tunneling - Static and automatic tunnelling of IPv6 packets over an IPv4 infrastructure
• NAT-PT - Static or Dynamic Network Address Translation - Protocol Translation

Dual Stack:

• Dual stack is the core technique which the other transition techniques are built upon.
• Dual Stack is the technique to allow an IPv4 and IPv6 address to be defined on the same network interface.

Tunnelling:

Static Tunnels, Point to Point Mechanisms.

• The advantage of doing static tunnels is that dynamic routing protocols can be run over the tunnel. There are 2 variants involved here for static tunnels:
• GRE - Default tunnel mode
• IPv6IP - Less overhead, no CLNS (IS-IS) transport Configuring static tunnels is a pretty straight forward process and the difference between configuring a GRE or a IPv6 IP tunnel is marginal.

Automatic tunnels, Point to Multipoint Mechanisms:

• It uses 6to4 site to site tunnel.
• It uses ISATAP host to host & host to router tunnel protocol (Within a Site).
• The advantage of automatic tunneling is that only one tunnel is needed (together with BGP or static routing) in order to establish a full mesh of connectivity.
• IPv6 prefix 2002: is reserved for the use of automatic tunneling. The IPv4 tunnel address is converted to Hex and pre-ended with 2002:.

6to4 Tunnels:

• The configuration of a 6to4 tunnel is needed when one IPv6 site has to be connected with another IPv6 site through an IPv4 infrastructure.
• The principle is simple as the mechanism to use is to create a tunnel interface. The core part is to calculate the two 48 bit IPv6 subnets to be used between the two sites.
• Earlier these subnets are calculated from the IPv4 IP address on both site of the point to point link.
• After these 2 subnets have been calculated on the sites, a static route to the remote 48 bit subnet pointing to the remote site’s 2002:: address is all what is needed to connect these two IPv6 sites together.

Teredo tunnelling:

• Instead of using routers to tunnel packets, Teredo tunneling has the hosts perform the tunneling. This requires the hosts to be configured with double stacks.

ISATAP Tunnels:

• ISATAP tunnels are needed when point to multipoint connections must be made within one site.
• The important difference here is in how the IPv6 endpoints addresses are calculated.

IPv6 translation:

• IPv6 translation schemes implement some form of packet-header translations between the IPv6 and IPv4 addresses.
• The goal is to translate packets with IPv6 addresses to those with IPv4 addresses, so that IPv6-only hosts can talk to the IPv4-only Internet.