【正文】 f addresses for us to use. IPv4 has only about billion addresses availablein theory, and we know that we don39。t even get to use all of those. There really are only about 250 million addresses that can be assigned to devices. Sure, the use of Classless InterDomain Routing (CIDR) and Network Address Translation (NAT) has helped to extend the inevitable dearth of addresses, but we will run out of them, and it39。s going to happen within a few years. China is barely online, and we know there39。s a huge population of people and corporations there that surely want to be. There are a lot of reports that give us all kinds of numbers, but all you really need to think about to convince yourself that I39。m not just being an alarmist is the fact that there are about billion people in the world today, and it39。s estimated that just over 10 percent of that population is connected to the Internetwow! That statistic is basically screaming at us the ugly truth that based on IPv439。s capacity, every person can39。t even have a puterlet alone all the other devices we use with them. I have more than one puter, and it39。s pretty likely you do too. And I39。m not even including in the mix phones, laptops, game consoles, fax machines, routers, switches, and a mother lode of other devices we use every day! So I think I39。ve made it pretty clear that we39。ve got to do something before we run out of addresses and lose the ability to connect with each other as we know it. And that something just happens to be implementing IPv6.The Benefits and Uses of IPv6So what39。s so fabulous about IPv6? Is it really the answer to our ing dilemma? Is it really worth it to upgrade from IPv4? All good questionsyou may even think of a few more. Of course, there39。s going to be that group of people with the timetested and wellknown resistance to change syndrome, but don39。t listen to them. If we had done that years ago, we39。d still be waiting weeks, even months for our mail to arrive via horseback. Instead, just know that the answer is a resounding YES! Not only does IPv6 give us lots of addresses ( x 1038=definitely enough), but there are many other features built into this version that make it well worth the cost, time, and effort required to migrate to it. Later in the chapter I39。ll talk about all that effort in the section called Migrating to IPv6. In it, I39。ll cover some of the transition types required to move from version 4 to version 6, and I promise you39。ll discover that the huge benefits of migrating will vastly outweigh any associated cons. Today39。s networks, as well as the Internet, have a ton of unforeseen requirements that simply were not considerations when IPv4 was created. We39。ve tried to pensate with a collection of addons that can actually make implementing them more difficult than they would be if they were required by a standard. By default, IPv6 has improved upon and included many of those features as standard and mandatory. One of these sweet new standards is IPSeca feature that provides endtoend security that I39。ll cover ili Chapter 14, Wide Area Networks. Another little beauty is known as mobility, and as its name suggests, it allows a device to roam from one network to another without dropping connections. But it39。s the efficiency features that are really going to rock the house! For starters, the header in an IPv6 packet have half the fields, and they are aligned to 64 bits, which gives us some seriously soupedup processing speedpared to IPv4, lookups happen at light speed! Most of the information that used to be bound into the IPv4 header was taken out, and now you can choose to put it, or parts of it, back into the header in the form of optional extension headers that follow the basic header fields. And of course there39。s that whole new universe of addresses ( x 1038) we talked about already. But where did we get them? Did that Criss AngelMindfreak dude just show up and, Blammo? I mean, that huge proliferation of addresses had to e from somewhere! Well it just so happens that IPv6 gives us a substantially larger address space, meaning the address is a whole lot biggerfour times bigger as a matter of fact! An IPv6 address is actually 128 bits in length, and no worriesI39。m going to break down the address piece by piece and show you exactly what it looks like ing up in the section IPv6 Addressing and Expressions. For now, let me just say that all that additional room permits more levels of hierarchy inside the address space and a more flexible address architecture. It also makes routing much more efficient and scalable because the addresses can be aggregated a lot more effectively. And IPv6 also allows multiple addresses for hosts and networks. This is especially important for enterprises jonesing for availability. Plus, the new version of IP now includes an expanded use of multicast munication (one device sending to many hosts or to a select group), which will also join in to boost efficiency on networks because munications will be more specific. IPv4 uses broadcasts very prolifically, causing a bunch of problems, the worst of which is of course the dreaded broadcast storman uncontrolled deluge of forwarded broadcast traffic that can bring an entire network to its knees and devour every last bit of bandwidth. Another nasty thing about broadcast traffic is that it interrupts each and every device on the network. When a broadcast is sent out, every machine has to stop what it39。s doing and respond to the traffic whether the broadcast is meant for it or not. But smile everyone: There is no such thing as a broadcast in IPv6 because it uses multicast traffic instead. And there are two other types of munication as well: unicast, which is the same as it is in IPv4, and a new type called anycast. Anycast munication allows the same address to be placed on more than one device so that when traffic is sent to one device addressed in this way, it is routed t