# Classful Subnet Masks and Binary Addressing

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9 hours 49 minutes
Difficulty
Beginner
CEU/CPE
10
Video Transcription
00:05
So in our previous section, we talked about the fact that there has to be a means of separating out the network ID from the host ID.
00:12
Remember, the network idea is going to be what narrows us down from anywhere in the world.
00:17
Basically, what's that going to do is allow traffic to get to your local router.
00:22
Once traffic find your local router,
00:24
then traffic has to find you specifically.
00:27
That's what the host portion of the address is.
00:30
So the network portion gets traffic near you. The host portion gets traffic to you.
00:36
You can't say that one is more important than the other. We need both of them.
00:40
The way that we determine what the network portion is from the host portion is we use something referred to as a sub net mask.
00:47
The only purpose of a sub net mask is to tell you what network portion is and tell you what host portion is.
00:54
A sub net mask is worthless without an I p. An I P address is worthless without a sub net mask. The two of them work together,
01:03
so let's take a basic I p address. Let's take this 114.25. 0.37 point eight
01:11
that I've used here
01:12
and say that I tell you to use a class A sub net mask.
01:15
Classy sub net mask 255.0 point 0.0.
01:21
What that tells me is the first octet of the I P address is network portion,
01:26
while the remaining zeros are host abortions.
01:29
For now, you can think of it as when there is a 255 that's part of the network ID.
01:34
And where there's a zero, it's part of the host ID.
01:38
So if I look at this I, p address and the subnet mask together,
01:42
the sub net mask tells me that the first octet is part of the network ID.
01:47
Everything else is to identify hosts.
01:49
So my network idea or network address in this case is 114.0 point 0.0.
01:57
The remainder the 25.37 point eight
02:00
is unique to the host.
02:04
Now, if I change the sub net mask, your network ID versus host ID is going to change.
02:09
So on Class B
02:12
255.255 point 0.0 you've got 114.25 point 00 as your network ID,
02:21
and then the remainder. The host ID is 37.8 is unique to the specific host.
02:28
Class C
02:29
255.255 point 255.0 1st three octaves of the I P address our network ID
02:36
so 114.27 point 37.0 and all that's left is for the host address is 0.8.
02:46
So where you see a 255 opted in the sub net masks tells you what to isolate from I p address as the network ID.
02:53
These are classical masks,
02:55
and you can see a Class A addresses this a Class B address? Is this a Class C address or C sub net mask? Is this
03:04
so It's either 255 or zero
03:07
in a little while, we'll look at this, but what you'll see is for separate Oct. It's whereas in reality, your system sees that as 32 bits.
03:16
Each octet is eight bits separated by a dot.
03:20
If we get into binary, what we'll see in a few minutes is that when you all have binary ones in a bit of data,
03:27
the value for that is 255
03:29
So what we really want to move towards thinking is that it's not so much to 55 is magical, but where there are binary ones, that's a portion of network ID
03:39
where there are portions of binary zeros. That's the host ID,
03:44
but we'll look at how to convert it to binary in just a minute.
03:50
When we look at binary addressing, I want you to disregard was on the screen and just think back to how you learn math
03:55
The way we use math and the numbering system we use is the decimal system. It's based on tense.
04:01
So at some point in time, your teacher probably came in, wrote a number on the board and said,
04:06
Here's how many ones you have here is how many times you have. Here's how many hundreds and thousands you have.
04:12
The reason we use a numbering system that starts with one then 10, then 101,000 and so on is because we're based on 10.
04:20
Those are units of 10,
04:23
but with binary were based on to each value is either one or zero.
04:30
So let's focus on what's in pink. In this chart.
04:33
If we're looking to express one byte of data, which is what we have here, if you count the individual rectangles and pink,
04:40
we have eight of them from one all the way to 1 28.
04:44
So we're showing what can be expressed in a byte of data.
04:46
So we start over to the far right with the Ones column.
04:50
The reason we start with ones is we start out with two to the power of zero.
04:55
Anything to the power of zero is one.
04:58
So the first place holder is one.
05:00
Then we go to the power of one, which is to
05:02
to the power of two is four to the power of three is eight.
05:06
The power of four is 16.
05:09
I think you can see we continue to the power of 56 and seven.
05:14
That's our framework for converting to decimal.
05:15
So let's see how this works.
05:18
Let's take the number 156.
05:21
We see that in decimal.
05:24
I want to figure out what it looks like in binary.
05:27
So when I convert to binary, I have to remember the fact that one means yes, zero means no,
05:33
we can think about it that way.
05:35
So I have to ask myself, Well, the number 1 28 fit into 1 56 and the answer is yes.
05:43
And if so, I have to account for that.
05:45
So I subtract out 1 28 and that leaves me with the remainder of 28.
05:50
Well, 64 fit into 28. No.
05:54
So that's zero.
05:55
Well, 32 fit? No. So that's zero.
05:59
Well, 16 ft? Yes. So we have one
06:01
now
06:02
28 minus 16 leaves me with 12.
06:05
Eight, will fit into 12 with the remainder of four,
06:09
or will fit into four. And there's nothing left over, so you finish it out with zeros.
06:15
So when I look at the number 1 56 in binary,
06:17
it's
06:18
10011100
06:24
Let's do another one.
06:25
Let's go down to the number 99
06:28
with the number 99. If we look at that in binary, 128 won't fit into 99.
06:33
So we get zero.
06:35
64 will fit. So we get one.
06:39
Subtract 64 we're left with 35.
06:43
Well, 30 to fit into 35.
06:45
Yes. So one
06:46
that leaves me with the remainder of three.
06:49
So 16, 8 and four won't fit.
06:53
But to will
06:55
we get a one for the to value? We have one left over.
06:59
So one in the ones place.
07:01
So 99 binary
07:03
01100011
07:08
Mhm.
07:09
Okay, so let's look at 255 in binary
07:13
255 in binary. If you go through the same process, you'll see 255 in binaries all once
07:19
that becomes important later, because what we're going to find is 255 or zero. That's too much all or nothing.
07:28
So what we'll do is take that number 255 for the sub net mask.
07:31
And rather than using all the bits as binary one,
07:34
I'm going to play around with that so that we would get a more efficient usage of our I p address space.
07:42
So for now, think about the classical I P addresses in the sub net masks Sub net Mask A, B and C.
07:48
Remember for later that 255 when it's used to indicate a network portion of an I P address, that number is special because of the fact that it's all binary ones.
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