Upgrading and Repairing Networks (5th Edition)

What's the Difference Between IPv4 and IPv6?

The IP protocol is a connectionless, unreliable protocol. TCP uses IP to establish sessions with remote computers and provides the reliability of the data transactions. IP, however, provides the hierarchical address space used by IPv4. Yet this address space is limited to fields in the IP datagram that are only 32 bits in length. When first created, it seemed like this address space would provide enough IP addresses to last for decades or more. After all, only government, educational facilities, and a few other institutions used what was then the ARPANET (the predecessor of the Internet). The address classes' original part of the IPv4 address space has pretty much been displaced by CIDR, to prevent wasting large ranges of addresses allocated to a single entity (such as class A networks).

IPv6 increases this 32-bit address space to 128 bits. At first glance, 32 bits versus 128 bits doesn't seem to be a big difference. When you consider the number of possible addresses that each of these bit ranges can provide, however, there is a tremendous difference. Fill a 32-bit field with all ones and you end up with a number just over 4 billion. A 128-bit field can provide a much larger number of possible addresses. The actual number of addresses, of course, depends on which bits are used to identify a network and which are used to identify a host on a network.

The address space that IPv4 enables can give us enough addresses to satisfy the demand today, especially when using NAT for LANs and using CIDR to reclaim wasted address space that was created by the original address classes. Yet the world of electronics today has changed the playing field. It's not just computers that need an IP address. Handheld devices, mobile phones, and other consumer devices will likely require an IP address in the near future. NAT might work well in a LAN or a small enterprise network, but when you consider that many wireless devices will roam from one provider to another, an assigned IP address becomes more important. NAT is performed at a local level, not a national or global one.

Expanding the IP address space is not the only feature that IPv6 gives to the Internet and your LAN or WAN. Other important features include the following:

• A simpler header format for the IP datagram, which makes it possible to create faster routing techniques implemented in hardware designs.

• Support for new extensions to the IP header, as well as a means to include future expansion for additional headers that may be created later.

• The replacement of certain options left over from the IPv4 specification, as well as new options, and, again, room for expansion of additional options as required in the future.

• The capability to specify which datagrams require special handling when it comes to flow control. This capability can enable real-time handling of a stream of IP datagrams (needed, for example, for real-time voice or video communication over an IP network), a feature usually accomplished by other protocols tunneling IP.

• Authentication and encryption capabilities to provide for a secure connection.

As you can see, there are many differences between the capabilities of IPv4 and those of IPv6. The remainder of this chapter will give you an idea of how this new functionality is accomplished.