2022年2022年计算机网络专业译文 .pdf

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1、1 专业译文原文：Cisco Certified Network Associate Study Guide.Sixth Edition的第 740-747 页（作者：Todd Lammle 出版社：Wiley Publishing,Inc.）英语原文：I hope youre ready to learn about the nuts and bolts of Internet Protocol version 6(IPv6),because youre going to get the rub on it in this chapter!You should have a solid ho

2、ld on IPv4 by now,but if you think you could use a refresher,just page back to Chapter 3,Subnetting,Variable Length Subnet Masks(VLSMs),and Troubleshooting TCP/IP.And if youre not crystal clear on the address problems inherent to IPv4,you really should review Chapter 11,Network Address Translation(N

3、AT)”People refer to IPv6 as the next-generation Internet protocol,and it was originally created as the answer to IPv4s inevitable,looming address-exhaustion crisis.Though youve probably heard a thing or two about IPv6 already,it has been improved even further in the quest to bring us the flexibility

4、,efficiency,capability,and optimized functionality that can truly meet our ever increasing needs.The capacity of its predecessor,IPv4,pales in comparison-and thats the reason it will eventually fade into history completely.The IPv6 header and address structure has been completely overhauled,and many

5、 of the features that were basically just afterthoughts and addendums in IPv4 are now included as full-blown standards in IPv6.Its seriously well equipped,poised,and ready to manage the mind-blowing demands of the Internet to come.I promise-really-to make this chapter pretty painless.In fact,you mig

6、ht even find your self actually enjoying it-I definitely did!Because IPv6 is so complex yet elegant,innovative and chock-full of features,it fascinates me like some weird combination of a brand-new Lamborghini and a riveting futuristic novel.Hopefully youll experience this chapter as the cool ride t

7、hat I did writing it!Why Do We Need IPv6?Well,the short answer is,because we need to communicate,and our current system isnt really cutting it anymore-kind of like how the Pony Express cant compete with airmail.Just look at how much time and effort weve invested in coming up with slick new ways to c

8、onserve bandwidth and IP addresses.Weve even come u p with Variable Length Subnet Masks(VLSMs)in our struggle to overcome the worsening address drought Its reality-the number of people and devices that connect to networks increases each and every day.Thats not a bad thing at all-were finding new and

9、 exciting ways to communicate to more people all the time,and thats a good thing.In fact,its a basic human need.But the fore cast isnt exactly blue skies and sunshine because,as I alluded to in this chapters introduction,IPv4,upon which our ability to communicate is presently dependent,is going to r

10、un out of addresses for us to use.IPv4 has only about 4.3 billion addresses available-in theory,and we know that we dont 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 Inter-Domain Routing(CIDR)and Network Addr

11、ess Translation(NAT)has helped to extend the inevitable dearth of addresses,but we will run out of them,and its going to happen within a few years.China is barely online,and we know theres a huge population of people and corporations there that surely want to 名师资料总结-精品资料欢迎下载-名师精心整理-第 1 页，共 15 页 -2 b

12、e.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 Im not just being an alarmist is the fact that there are about 6.5 billion people in the world today,and its estimated that just over 10 percent of that population is conne

13、cted to the Internet-wow!That statistic is basically screaming at us the ugly truth that based on IPv4s capacity,every person cant even have a computer-let alone all the other devices we use with them.I have more than one computer,and its pretty likely you do too.And Im not even including in the mix

14、 phones,laptops,game consoles,fax machines,routers,switches,and a mother lode of other devices we use every day!So I think Ive made it pretty clear that weve 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 happe

15、ns to be implementing IPv6.The Benefits and Uses of IPv6 So whats so fabulous about IPv6?Is it really the answer to our coming dilemma?Is it really worth it to upgrade from IPv4?All good questions-you may even think of a few more.Of course,theres going to be that group of people with the time-tested

16、 and well-known resistance to change syndrome,but dont listen to them.If we had done that years ago,wed 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(3.4 x 1038=definitely eno

17、ugh),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 Ill talk about all that effort in the section called Migrating to IPv6.In it,Ill cover some of the transition types required to move from ver

18、sion 4 to version 6,and I promise youll discover that the huge benefits of migrating will vastly outweigh any associated cons.Todays networks,as well as the Internet,have a ton of unforeseen requirements that simply were not considerations when IPv4 was created.Weve tried to compensate with a collec

19、tion of add-ons 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 IPSec-a feature that provides end-to-end

20、security that Ill 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 its the efficiency features that are really going to rock the house!For starters,the h

21、eader in an IPv6 packet have half the fields,and they are aligned to 64 bits,which gives us some seriously souped-up processing speed-compared 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 pa

22、rts of it,back into the header in the form of optional extension headers that follow the basic header fields.And of course theres that whole new universe of addresses(3.4 x 1038)we talked about already.But where did we get them?Did that Criss Angel-Mindfreak dude just show up and,Blammo?I mean,that

23、huge proliferation of addresses had to come from somewhere!Well it just so happens that IPv6 gives us a substantially larger address space,meaning the address is a whole lot bigger-four times bigger as a matter of fact!An IPv6 address is actually 128 bits in length,and no worries-Im going to break d

24、own the address piece by piece and show you exactly what it looks like coming up in the section IPv6 Addressing and Expressions.For now,let me just say that all 名师资料总结-精品资料欢迎下载-名师精心整理-第 2 页，共 15 页 -3 that additional room permits more levels of hierarchy inside the address space and a more flexible a

25、ddress 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 n

26、ow includes an expanded use of multicast communication(one device sending to many hosts or to a select group),which will also join in to boost efficiency on networks because communications will be more specific.IPv4 uses broadcasts very prolifically,causing a bunch of problems,the worst of which is

27、of course the dreaded broadcast storm-an 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

28、sent out,every machine has to stop what its 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 communication as well:unicast,which is

29、 the same as it is in IPv4,and a new type called anycast.Anycast communication 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 to the nearest host that shares the same address.This is just the beginning-well g

30、et more into the various types of communication in the section called Address Types”IPv6 Addressing and Expressions Just as understanding how IP addresses are structured and used is critical with IPv4 addressing,its also vital when it comes to IPv6.Youve already read about the fact that at 128 bits,

31、an IPv6 address is much larger than an IPv4 address.Because of this,as well as the new ways the addresses manage.But can be used,youve probably guessed that IPv6 will be more complicated to no worries!As I said,Ill break down the basics and show you what the address looks like,how you can write it,a

32、nd what many of its common uses are.Its going to be a little weird at first,but before you know it,youll have it nailed!So lets take a look at Figure 13.1,which has a sample IPv6 address broken down into sections.FIGURE 13.1 IPv6 address example So as you can now see,the address is truly much larger

33、-but what else is different?Well,first,notice that it has eight groups of numbers instead of four and also that those groups are separated by colons instead of periods.And hey wait a secondthere are letters in that address!Yep,the address is expressed in hexadecimal just like a MAC address is,so you

34、 could say this address has eight 16-bit hexadecimal colon-delimited blocks.Thats already quite a mouthful,and you probably havent even tried to say the address out loud yet!One other thing I want to point out is for when you set up your test network to play with IPv6,because I know youre going to w

35、ant to do that.When you use a web browser to make an HTTP connection to an IPv6 device,you have to type the address into the browser with brackets around the literal address.Why?Well,a colon is already being used by the browser for specifying a port 名师资料总结-精品资料欢迎下载-名师精心整理-第 3 页，共 15 页 -4 number.So b

36、asically,if you dont enclose the address in brackets,the browser will have no way to identify the information Heres an example of how this looks:http:/2001:Odb8:3c4d:0012:0000:0000:1234:56ab/default.html Now obviously if you can,you would rather use names to specify a destination(like ),but even tho

37、ugh its definitely going to be a pain in the rear,we just have to accept the fact that sometimes we have to bite the bullet and type in the address number So it should be pretty clear that DNS is going to become extremely important when implementing IPv6.Shortened Expression The good news is there a

38、re a few tricks to help rescue us when writing these monster addresses.For one thing,you can actually leave out parts of the address to abbreviate it,but to get away with doing that you have to follow a couple of rules.First,you can drop any leading zeros in each of the individual blocks.After you d

39、o that,the sample address from earlier would then look like this:2001:db8:3c4d:12:0:0:1234:56ab Okay,thats a definite improvement-at least we dont have to write all of those extra zeros!But what about whole blocks that dont have anything in them except zeros?Well,we can kind of lose those too-at lea

40、st some of them.Again referring to our sample address,we can remove the two blocks of zeros by replacing them with double colons,like this:2001:db8:3c4d:12:1234:56ab Cool-we replaced the blocks of all zeros with double colons.The rule you have to follow to get away with this is that you can only rep

41、lace one contiguous block of zeros in an address.So if my address has four blocks of zeros and each of them were separated,I just dont get to replace them all;remember the rule is that you can only replace one contiguous block with a double colon.Check out this example:2001:0000:0000:0012:0000:0000:

42、1234:56ab And just know that you cant do this:2001:12:1234:56ab Instead,this is the best that you can do:2001:12:0:0:1234:56ab The reason why the above example is our best shot is that if we remove two sets of zeros,the device looking at the address will have no wav of knowing where the zeros go bac

43、k in Basically,the muter would look at the incorrect address and say,Well,do I place two blocks into the first set of double colons and two into the second set,or do I place three blocks into the first set and one block into the second set?And on and on it would go because the information the muter

44、needs just isnt there.Address Types Were all familiar with IPv4s unicast,broadcast,and multicast addresses that basically define who or at least how many other devices were talking to.But as I mentioned,IPv6 adds to that trio and introduces the anycast.Broadcasts,as we know them,have been eliminated

45、 in IPv6 because of their cumbersome inefficiency.So lets find out what each of these types of IPv6 addressing and communication methods do for us.Unicast Packets addressed to a unicast address are delivered to a single interface.For load balancing,multiple interfaces can use the same address.There

46、are a few different types of unicast 名师资料总结-精品资料欢迎下载-名师精心整理-第 4 页，共 15 页 -5 addresses,but we dont need to get into that here.Global unicast addresse s These are your typical publicly mutable addresses,and theyre the same as they are in IPv4 Link-local addresses These are like the private addresses i

47、n IPv4 in that theyre not meant to be routed.Think of them as a handy tool that gives you the ability to throw a temporary LAN together for meetings or for creating a small LAN thats not going to be routed but still needs to share and access files and services locally.Unique local addresses These ad

48、dresses are also intended for non-routing purposes,but they are nearly globally unique,so its unlikely youll ever have one of them overlap.Unique local addresses were designed to replace site-local addresses,so they basically do almost exactly what IPv4 private addresses do-allow communication throu

49、ghout a site while being mutable to multiple local networks.Site-local addresses were denounced as of September 2004.MulticastAgain,same as in IPv4,packets addressed to a multicast address are delivered to all interfaces identified by the multicast address.Sometimes people call them one-to-many addr

50、esses.Its really easy to spot a multicast address in IPv6 because they always start with FF.Ill get into greater detail about multicast operation in the section How IPv6 Works in an Internetwork.AnycastLike multicast addresses,an anycast address identifies multiple interfaces,but theres a big differ