IPv6 Addressing and Subnetting

It’s easy to be a little intimidated when you come across an IPv6 address for the first time in its fully expanded form. First of all, at 128 bits in length, an IPv6 address is a full four times longer (in bits) than its IPv4 equivalent. Because of their length, IPv6 addresses are represented in hexadecimal rather than the common dotted decimal notation you are familiar with from IPv4. However, before you start worrying about endlessly typing (and calculating) hexadecimal characters on each and every host on your network, it’s worth noting that IPv6 includes a variety of methods to make address deployment easier for administrators, features that I’ll cover in the IPv6 operation section below.

Beyond handling addresses differently, IPv6 also changes the method by which a network is subnetted. If anything, this method is actually simpler than with IPv4, because an IPv6 address includes a section to define a subnet number directly within the address itself. Once you understand where that field is, determining the subnet to which a host belongs is as easy as just reading and matching the number listed.

In this section I’ll walk you through some of the basic of IPv6 addressing and subnetting, including how IPv6 addresses are formatted and represented, how subnets are determined, how the address space itself is allocated, and the different types of addresses and transmission methods that exist.

Introduction to IPv6

Before getting into the technical details of this new version of IP, a simple question must be answered – why does the world need a new version of IP at all? There are many answers to this question, but the most basic reason involves the rapid depletion of the IPv4 address space. If you’ll recall from earlier in this chapter, IPv4 uses 32-bit addressing, and this limits the total number of IP addresses available for issue. When the Internet Protocol was first defined, nobody envisioned the phenomenal growth that defines the Internet we know today. As such, what was originally considered an almost endless supply of address space is now challenged as increasingly greater numbers of devices are connected to the Internet. Consider examples like cellular phones, PDAs, and other embedded systems, and it’s easy to see how the need for more IP address space is upon us.

Note: Talking about IPv6 begs a simple question – whatever happened to IPv5? The answer to that is that IPv5 is actually defined, although for a different purpose. IPv5 defines an experiment protocol that was originally developed to provide quality of service (QoS) features on IPv4 networks. You don’t need to know anything about IPv5 for the CCDA exam, but hopefully this helps to clear up what might have been a nagging thought. To that end, it’s also an interesting little piece of tech trivia!

A number of techniques have been developed and implemented to help slow down the need to deploy a new version of IP, and of these, network address translation (NAT) is clearly the most popular. Earlier in this chapter we also looked at the three ranges of IP addresses set aside for private addressing. While this technique has reduced the need for public IP addresses, it is still not a proper solution, and does little to address the foreseeable growth of the Internet in the longer term. Quite simply, a larger IP address space is required, and IPv6 addresses this issue with a 128-bit address space. At this time, a 128-bit address space would be capable of providing more than a thousand IPv6 addresses for each man, woman, and child on the planet.