lair to or your data link. And if I were you guys, I would get in the habit of moving away from calling layers by their name and moving to words, calling them by their number. Because when you do go apply for a job, a person interviewing you can tell how experienced you are
by whether you use the name or the number. If you say layer to more experienced people, just say later, too. If you continually say Dada Linklater Datalink clear the person interviewing you Will Gardner that you're not that experience that you passed your C C in a few days ago
on the immediate link between two devices or the immediate, as the name say's the immediate data link.
So layer to once again
controls communication between one device and the next immediately connected device.
So if I were to draw a pictorial representation off it
and I'm going to draw a circle in this class, every time I draw a circle, it means I'm drawing a router. And let's say
this router is connected to another router,
and whenever you see a thunderball being drawn in the Siskel world, it means this is a serial connection. Your cereal connections on a router, I your wide area links. So we're going to say this
port on this router is cereal zero slash zero slash zero,
and I'm gonna go into the naming conventions for routers in a little bit.
It doesn't pretend to this part of the lecture. So rest assured, I will be covering how Cisco names and there is a convention. Let's hear the other one. Also a serial zero slash zero slash zero
This immediate link between these two devices
this immediate connection between these two devices is your local datalink. So lair to your dead Ellen Clear controls communications
on the immediate link between two devices. Now, we understand that the data link lier
controls communication on the immediate link immediate physical link,
which I will change the rules in a little bit. But for now, the immediate physical link between two devices
now data and Lair One. If you were to think about the structure of data at layer one
and if I ask you a question,
what does the data at layer one looked like?
Your answer should be that the day that layer one is simply electricity. Electrical signals or changes in voltage
now at layer to the data has been put into a set structure.
So at every layer passed the physical Lear,
the data is put into a set structure and this set structure is called your P D. You or your protocol data unit
at layer to the protocol did a unit is called a friend.
Now what does the frame look like?
Well, before we get to what a frame looks like
when there is data transmission between two devices just like when there is
data transmission between your home and somebody you're mailing a letter to, you need two pieces of information.
You need your own house address,
which would be your source address,
and you need a destination address, which would be the address of the person You're sending the letter too
in a similar manner at layer to you need an address ing scheme
the moves common address ing scheme that we see it later to and there are others, but they coincide with
a litter part of the class and I will bring them up when pertinent.
But the data stream at layer to eat a source address from which the stream is sent from,
and a destination address, which the stream is sent to. The most common of these addresses is your media access control address or your
Now what does the Mac address look like?
A Mac address is 48 bits in length, and it's written in Hexi Decimal.
Now, I'm not going to assume that people here listening to this lecture know what Hexi decimal numbers are
so real quick. What are Hexi? Decimal numbers are in short, what are X numbers? Next numbers range between one and nine
and then a through F. So X numbers go one through nine, then a through F.
So if I wanted to say 10
in Hexi Decimal, I would just say Aye.
If I wanted to say 11 in Hexi Decimal, I would just say, B,
I hope I got my hex numbers across to you guys. Now I can get to what a Mac address looks like. Remember, Mac Address is 48 bits in length
and is written in Exodus to an example off. A Mac address would be 00 to a A B C D
this? How does this number or this Mac address equate to 48?
Each one of these hex character's
So zero is actually representing four bits.
is actually representing four bit
it actually means 10 inaccessible. So if I were to expand out into its four constituents, constituents
bit sorry for the mispronunciation. Let's do that.
Four bets. Okay, so we dropped four lines representing four bets,
and each one of these bits has a specific value attached to it. And this value never changes
from the very right hand side. You start with a number one,
then you double it. You get to you, double it, you get four, you double it and you get the number eight.
How do I get a out of these four bits? It actually stands for 10. Right?
So the eight bit is on and the two bit is on the four, and the one bits are turned off.
Now, I'm gonna raise this part because I need to explain other stuff about the Mac address. The 1st 24 bits in the Mac address
where does the line will do I draw the line for the 24 bits. That's his 16 bits.
17 18 1920 2122 23 24. So right at the beep,
the 1st 24 bits off a Mac address
are your Oh, you I feel,
or the organizationally unique identify WR
the oil you I field is designated by a governing body, and it cannot be changed.
And this is vendor specific. So each vendor, for example, Cisco, Microsoft, Dell,
each one of these renders are assigned several or you guys and these Oh, you guys cannot be changed
off a Mac address. The vendor can do whatever they want with them.
So the first Mac address created by Cisco ever was probably a vendor specific or you I field and then 000001 And they went from the 0002 So the tension around her they made was probably 00
000 a and so on and so forth.
off a Mac address, the seventh most significant bit. And whenever I use the language, most significant, I mean all the way to my left over here
or all the way to your right,
the first in the 1st 4 in the second Corvettes.
So the seventh Morse most significant bet
which would reside in here because four bits here than 56 and seventh bit, which would reside in here, is called the UL bit or the Universal Local bit.
If done bit is turned off, which it is over here.
If that *** is turned off,
it means that this Mac address is universally unique.
They won't be another Mac address like this, in theory
goes even then, sometimes you run across Mac addresses that are the same, but it doesn't happen often.
But in theory, if this bit is turned off, the seventh most significant bit is turned off. It means that this Mac address,
quite possibly or should be universally unique.
this bit is turned on.
For example, you. Sometimes you have the option of changing the Mac address on a Cisco device. You have the option of actually typing in a Mac address. Then, if it is locally assigned if that mackerel Mac address has been tampered with, so to say the seventh most significant bit would have to be turned on.
So when the Brit is on, it means this address has been assigned locally. So let's go ahead and turn that bit on and see what happens.
The seventh bit would reside in him.
Remember, at this end you have four bits
that denotes the zero so four bits for this 0/5 bit, six bit seventh.
well, I won't write it because I have no space. But this bit the to value bit is your seventh.
If you turn this on and by default, it was off. Then you get 02 to a
and then a B C D. And 1234
So if the seventh most significant bit is off,
it means address is unique.
If it's on, that means it has been assigned locally