IPv4 has been in use as the predominant protocol on the Internet for nearly three decades. It was originally defined in 1981 in a technical specification known as RFC 791. IPv4 is a connectionless protocol that operates at the Network Layer (Layer 3) of the OSI model. IPv4 is the foundation of the TCP/IP protocol suite as it exists today.

IPv4 was designed with several assumptions in mind, many of which have been proved inaccurate, grossly overestimated, or simply nonapplicable. While IPv4 has been the predominant protocol on the Internet, a replacement is long overdue. Some of the key issues of concern are exhausted address space of only 32 bits, subnetting complexity, and a lack of integrated security. Some of these issues have been minimized with the advent of network address translation (NAT), classless inter-domain routing (CIDR), and Internet Protocol Security (IPSec). But in spite of these advancements, IPv4 is being replaced with IPv6.

IPv6 was defined in 1998 in RFC 2460. The new version was designed specifically as the successor to IPv4, mainly due to its dwindling availability, which was foreseen in the late 1990s. IPv6 uses a 128-bit address, which is significantly larger than IPv4’s 32-bit addressing. FIGURE 3-17 compares an IPv4 address to an IPv6 address.

Additionally, changes to subnetting, address assignment, and packet headers, and simpler routing processing, make IPv6 much preferred over its predecessor. Some benefits of IPv6 include:

• Increased address space—IPv6 supports 340 undecillion (that’s 1 followed by 36 zeros) IP addresses for network devices.
• More efficient routing—The routing functionality of IPv6 has been enhanced.
• Reduced management requirement—A more robust protocol, IPv6 requires less management. In contrast, IPv4 required significant management due to requirements added after the introduction of the protocol.
• Better quality of service (QoS) support for all types of applications—Support for QoS is built into IPv6, whereas it was an add-on in IPv4.
• Security—IPv6 includes a native information security framework (IPSec) that provides for both data and control packets.
• Plug-and-play configuration with or without DHCP—IPv6 permits hosts to automatically configure during connection to an IPv6 network by querying the local routers with a multicast message. If the routers are configured correctly, a response with the appropriate configuration information is sent. If this mechanism is not appropriate for a specific environment or application, support is available for an IPv6 version of DHCP. It also supports static configurations.

Three mechanisms currently exist for the transition from IPv4 to IPv6. It is important to understand that when the IPv6 standard appeared, the expectation was that the two protocols would need to coexist in the network for 20 to 30 years, allowing for a gradual transition period. The migration strategies are:

• Dual-stack—With this transition solution, both IPv4 and IPv6 protocol stacks coexist in the same terminal or network equipment. This allows the network to communicate using both protocols, but it adds significant overhead to the network infrastructure.
• Tunneling—This solution allows two IPv6 hosts to create a tunnel for traffic between two IPv6 hosts through an IPv4 network, or vice versa, as IPv4 is phased out. This could add significant configuration overhead.
• Translation—This solution enables an IPv4 host to talk to an IPv6 host. This solution will require additional development, as currently IPv6 does not include this capability.