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- So now that we've seen

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that the IPv6 address
itself has been changed,

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it's much larger, and it's now
represented in hexadecimal,

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let's see what other things
about IPv6 they modified.

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So I'm gonna zoom in on this.

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So, up here we have
our IP version 4 Header

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that we've already looked at.

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And we talked about all these things:

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the Version, IP Header
Length, Identification, Flags,

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and Fragment Offset,
we've looked at all that.

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So now look at the IPv6 Header.

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And we can see there are
some things that are the same

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and some things that are different.

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First of all, what's the same.

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Well, there is a Version field in there.

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Of course in IP version 4 this
had the number four in it,

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and now this has the number six in it.

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Okay, what else is similar.

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Well, we certainly have the Addresses.

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Everything else is either
gone, new, or renamed.

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So let's look at each of these things.

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First of all, Traffic Class.

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This is the exact same thing
as the Type of Service field.

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They just renamed it.

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Still the same size is used

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to identify the priority of a packet,

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it's just now called Traffic Class.

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And Total Length here, same
thing as Payload Length,

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so nothing is different about that.

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The Next Header field

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is the same thing as the Protocol field.

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They just renamed it.

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Here is a review question:

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In IP version 4, what number
would be in a Protocol field

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if this IPv4 packet was carrying
a TCP segment behind it?

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That would the number six, right?

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Because TCP was six,

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well, that's all we'd
have right here as well,

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the number six.

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What number would be in this field

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if it was carrying a
UDP datagram behind it?

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That would be the number 17.

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17 would be in here.

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So, the numbers are the same,

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just the name of the field is different.

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And the Time to Live right here,

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that's just renamed Hop Limit.

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Same exact functionality,
just a different name.

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Now, there is something
that's brand new: Flow Label.

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This is something that's new in IPv6.

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The idea behind the Flow Label,

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and honestly I'm not sure
as of right now, 2016,

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how many operating systems
actually make use of this field,

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but the theory was that
every unique flow of traffic

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from your laptop would
have a unique Flow Label.

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In other words, if I have
my internet browser open,

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I've got a tab open to ine.com,

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all the packets from my
laptop going to ine.com

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would have a unique Flow Label.

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If I open up a second
tab, going to google.com,

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well, that's another,
different unique flow.

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So my operating system
would put a different number

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in there, in the Flow Label.

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And the idea was,

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this just gives us one more
ability to track packets

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that we didn't have before.

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Now I don't know if
anybody actually uses that,

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but that's what the Flow Label is for.

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Now notice that there are several things

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that are conspicuously
absent from the IPv6 Header.

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For example, the IP Header
Length field, that's gone.

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The reason why the IP Header Length field

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was necessary in IP version 4

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was because of the presence of Options.

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There may be Options,
there may not be Options;

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if there were Options, who
knows how many there could be.

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So the size of this header was variable

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based on the Options.

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Well, the Options don't exist
anymore, they are not in here.

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So, I shouldn't say they don't exist,

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but they are no longer considered
part of the IPv6 Header.

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So, now anything that would be an option,

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has been given its own unique number,

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and it'll show up here,
in the Next Header field.

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So basically what you are looking at is,

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if this is my v6 Header,

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and let's say I have two Options,

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and then I have my TCP Header after that,

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oops, that doesn't look very good.

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I'll just leave it alone, TCP.

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Okay, well, this option
here, whatever it might be,

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will have a certain number.

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Let's say that this is option number 50.

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So in the Next Header field, right here,

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we would have the number 50,

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which tells us that that's next.

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00:04:48,148 --> 00:04:51,991
Now, just like the IPv6 Header
has a Next Header field,

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each one of these, these are
called Extension Headers,

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they also have Next Header fields.

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So if the second option
was option number five,

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we'd see the number five here.

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This option would say, "Okay,
after you're done with me,

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"the Next Header will be number five."

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00:05:09,521 --> 00:05:13,628
And then in this one right
here, you would see six,

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saying, "The next thing after me is TCP."

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So the initial IPv6 Header itself

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is always the exact same size.

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And the Next Header field
indicates what's coming next:

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is it an option coming next,

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or is it TCP, UDP, or something else.

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And then we've got all
of these right here.

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These have now been given
their own additional header.

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I'm not sure what the number is,

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but there is a Next Header field,

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00:05:42,963 --> 00:05:45,594
there is a Next Header number that says,

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"What you're gonna see
next is fragmentation,

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Identification, Flags,
and Fragment Offset."

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So if a packet is fragmented,
there will be something else

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inserted here after the
Destination Address,

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and whatever that number is

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for containing these three
fields will be put right here.

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I don't know what it is,

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00:06:03,279 --> 00:06:06,051
and you're never gonna be tested on that.

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And Header Checksum is missing.

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If you recall, what is a checksum?

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A checksum was a way of detecting

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if there were errors in something.

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Detecting if when I
receive a stream of bits,

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if those bits have been changed

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since they left the originator.

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Well, designers of IPv6,
they said, "You know what?

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"Most data-link layer protocols,
most Layer 2 protocols

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"already have checksums in place.

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00:06:32,944 --> 00:06:37,179
"And UDP and TCP have checksums in place.

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00:06:37,179 --> 00:06:40,934
"So why do we need to
do it again at Layer 3?

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00:06:40,934 --> 00:06:42,982
"It seems like kind of overkill."

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So, they got rid of it.

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And the last thing I want to talk about

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is a term that you will hear
frequently, called dual stack.

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What is dual stack?

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00:06:53,695 --> 00:06:57,778
Right now, as people are
still migrating to IPv6,

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you're not really gonna
find networks that say,

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"Oh, okay, let's strip off
all of our IPv4 addresses

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00:07:03,475 --> 00:07:05,795
"and replace it with IPv6."

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00:07:05,795 --> 00:07:07,803
Most people don't move in that direction.

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00:07:07,803 --> 00:07:10,763
What they do is they take
a device, like a web server

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or an email server that's
currently running IPv4,

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and they add IPv6 on top of it.

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00:07:17,297 --> 00:07:19,098
So that device can actually do both:

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it'll have an IPv4
address on its net card,

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and it'll also have an IPv6
address on its net card.

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When a device has both v4 and v6,

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00:07:27,679 --> 00:07:30,897
we call it a dual-stacked device.

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So whenever you see that term, dual stack,

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now you know, "Oh, that means
it's running IPv4 and IPv6."