On today’s Internet, IPv4 has the following disadvantages:
• Limited address space. The most visible and urgent problem with using IPv4 on the modern Internet is the rapid depletion of public addresses. Due to the initial address class allocation practices of the early Internet, public IPv4 addresses are becoming scarce. Organizations in the United States hold most public IPv4 address space worldwide. This limited address space has forced the wide deployment of network address translators (NATs), which can share one public IPv4 address among several privately addressed computers. NATs have the side effect of acting as a barrier for server, listener, and peer-to-peer applications running on computers that are located behind the NAT.
Although there are workarounds for NAT issues, they only add complexity to what should be an end-to-end addressable global network.
• Flat routing infrastructure. In the early Internet, address prefixes were not allocated to create a summarizable, hierarchical routing infrastructure. Instead, individual address prefixes were assigned and each address prefix became a new route in the routing tables of the Internet backbone routers. Today’s Internet is a mixture of flat and hierarchical routing, but there are still more than 85,000 routes in the routing tables of Internet backbone routers.
• Configuration. IPv4 must be configured, either manually or through the Dynamic Host Configuration Protocol (DHCP). DHCP allows IPv4 configuration administration to scale to large networks, but you must also configure and manage a DHCP infrastructure.
• Security. Security for IPv4 is specified by the use of Internet Protocol security (IPsec). However, IPsec is optional for IPv4 implementations. Because an application cannot rely on IPsec being present to secure traffic, an application might resort to other security standards or a proprietary security scheme. The need for built-in security is even more important today, when we face an increasingly hostile environment on the Internet.
• Prioritized delivery. Prioritized packet delivery, such as special handling parameters for low delay and low variance in delay for voice or video traffic, is possible with IPv4. However, it relies on a new interpretation of the IPv4 Type Of Service (TOS) field, which is not supported for all the devices on the network. Additionally, identification of the packet flow must be done using an upper layer protocol identifier such as a TCP or User Datagram Protocol (UDP) port. This additional processing of the packet by intermediate routers makes forwarding less efficient.
• Mobility. Mobility is a new requirement for Internet-connected devices, in which a node can change its address as it changes its physical attachment to the Internet and still maintain existing connections. Although there is a specification for IPv4 mobility, due to a lack of infrastructure, communications with an IPv4 mobile node are inefficient.
All of these issues and others prompted the Internet Engineering Task Force (IETF) to begin the development of a replacement protocol for IPv4 that would solve the problems of IPv4 and be extensible to solve additional problems in the future. The replacement for IPv4 is IPv6.
IPv6 solves the problems of IPv4 in the following ways:
• Huge address space. IPv6 addresses are 128 bits long, creating an address space with 3.4 × 1038 possible addresses. This is plenty of address space for the foreseeable future and allows all manner of devices to connect to the Internet without the use of NATs. Address space can also be allocated internationally in a more equitable manner.
• Hierarchical routing infrastructure. IPv6 addresses that are reachable on the IPv6 portion of the Internet, known as global addresses, have enough address space for the hierarchy of Internet service providers (ISPs) that typically exist between an organization or home and the backbone of the Internet. Global addresses are designed to be summarizable and hierarchical, resulting in relatively few routing entries in the routing tables of Internet backbone routers.
• Automatic configuration. IPv6 hosts can automatically configure their own IPv6 addresses and other configuration parameters, even in the absence of an address configuration infrastructure such as DHCP.
• Required support for IPsec headers. Unlike IPv4, IPv6 support for IPsec protocol headers is required. Applications can always rely on industry standard security services for data sent and received. However, the requirement to process IPsec headers does not make IPv6 inherently more secure. IPv6 packets are not required to be protected with Authentication Header (AH) or Encapsulating Security Payload (ESP).
• Better support for prioritized delivery. IPv6 has an equivalent to the IPv4 TOS field that has a single interpretation for nonstandard delivery. Additionally, a Flow Label field in the IPv6 header indicates the packet flow, making the determination of forwarding for nondefault delivery services more efficient at intermediate routers.
• Support for mobility. Rather than attempting to add mobility to an established protocol with an established infrastructure (as with IPv4), IPv6 can support mobility more efficiently.
Source of Information : Microsoft Press Windows Server 2008 TCP IP Protocols and Services