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All right, so in the last lecture, we talked about the network layer, the data like layer, you learned

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about Mac addresses and the importance of the different jobs that are necessary in order to deliver

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frame or even a packet end to end.

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Now we're going to dig into something.

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And, you know, something really freaks a lot of people out because there's a lot of perhaps it's a

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lot of steps and it doesn't really seem relevant.

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Like who said.

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That's right.

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You just go online and type in an IP address and get the subnets that are inside of that particular

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address as an ethical hacker, a penetration test or whatever, you absolutely need to know how to subnet

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because you need to know which network you're on.

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You need to know if you're in scope.

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And also it'll help you determine to help you sort of diagnose why a particular connection is failing.

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Maybe you're trying to pivot to another system in the target users environment.

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And it's felt like for some reason that could be because they're on another network or VLAN or even

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a private plan which has different controls and technical restrictions in place, which is preventing

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lateral movement.

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So let's just get right into this.

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And I think the easiest way to get into subletting is just to look at an IP address.

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OK, so we open a command prompt.

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I just typed IP config and you can immediately see this beautiful trifecta.

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Right.

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So for Ethernet zero, which is my current Ethernet adapter, here's my IP address.

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Ten three zero thirty four is my subnet mask.

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In his malleefowl gateway, OK, we're going to get into what this means in detail, but Ticino just

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off the top, this mask is really, really important because it tells the computer when it's time to

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send a packet to the default gateway.

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It tells the computer basically which network a particular asset resides on, you know, if you try

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to ping a host that is in a different network, then it's going to have to go to the default gateway.

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So the default gateway getting a look at its routing table and try to figure out which interface it

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put forward that frame out of in order to get it to the next top.

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So I know that sounds a little bit confusing.

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A mysterious let's just get right into this and how is all this stuff works.

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So let's just draw this IP out.

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Got 10.

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Dot three, dot zero, dot thirty four.

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OK, and this is how we see it, right?

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These are how this is how humans see IP addresses as a human.

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We see this number.

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Right.

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And that's not exactly how the computer sees it.

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The computer doesn't see a decimal number like this.

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The computer sees a binary number.

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And so you need to understand what these numbers are like in binary.

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But before you can do that, you need to know the place value.

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So an IP address consists of a bit, which is either on or off.

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And there are eight bits in a bite.

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This is one bite.

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This is one bite.

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This is one bite.

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Is another bite.

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Four bites.

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Tasty.

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All right.

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So it's actually a bite.

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B, why t eat a bite?

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That's the correct way to refer to it.

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Then you really need to know here is that this bite is represented in binary with various place values.

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So what you have actually is you have a one place, you have a tooth place, a four eight.

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I'm just doubling it.

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16, 32, 64.

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One eight, OK, and these are the place values.

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That all the bits will fit into.

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Make a little table here.

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So what we can do is we look at each one, let's start with zero, because it is the easiest to understand

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zero in binary, it's just zero zero zero zero zero zero zero zero.

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Right.

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I mean, there should be no surprises there.

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That's why I actually started there.

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Now, what about the number 34?

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What would that be?

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Number three for us?

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A little bit different.

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So we need to do is we need to convert this number to binary to see it's 34 right there.

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No, 128 and 34.

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There are no 64 and 34.

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There's 132.

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So now that means there's just two left, there's no sixteen's and two there's no eight and two there's

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no four and two and there's one two into this number is 34 and binary, OK?

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And that's what the computer sees with its last octet.

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And of course, you have the number three, but you can see these last two bits, some to make three

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to one plus two or three.

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So that's number three.

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And very and then lastly, you have the number 10.

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And you can just add eight plus two to make ten.

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And I feel the rest of my Xeros.

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So now that we figured all this out, we can actually trace this all and show you what it looks like,

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a Pinery.

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Right.

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So we figured out, Tim.

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We figured out three.

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We could have zero.

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And finally, 34.

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And by the way, while I'm doing this, I'm counting in my head, I'm saying 128, no.

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64, no.

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32, yes, 16 no.

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You know, when I say 32, that means only two.

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Left, right.

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So I'm saying how many sixteen's are in it to none?

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How many eights are in it to none?

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How many forces are in it to none?

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How many twos aren't it to one?

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Nothing less.

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So that should be this number in binary.

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And what we can do is check it just to open up the calculator.

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Right.

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So I can go down here.

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What I'll do is I'll change this into.

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And Billy Programer does it, yeah, there we go.

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So we can now type 10 and you can see ten in binary is one zero one zero leading zeros don't count.

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Of course, we can go back here is a three.

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Zero zero one one zero, obviously, will be all zeros.

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And finally, we clicked decimal to make your own decimal.

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We hit thirty four.

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And you can see.

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Everything matches up.

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OK, so now what do we do with this?

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Well, you may recall that in this case, we had a 24 mask, a 24 bed mask, right.

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And here was the mask.

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I'll give you the mask again.

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255, 255, 35, that zero, also known as a slash 24 y, because the first 24 bits are all ones, so

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a mask basically has to have contiguous bits, right?

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So by contiguous, I mean that the bits are next to each other.

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There's no breaks, there's no change in the digits.

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They're all ones consecutively.

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So this mask.

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255 represents one, two, three, four, five, six, seven, eight, all the bits are on in this byte.

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One, two, three, four, five, six, seven, eight, all the bits are on in the second octet, third

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octet, one, two, three, four, five, six, seven, eight, and then finally the last octet.

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Has nothing enabled, right?

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So what this is from the computer's perspective, and this is key, guys, this is key.

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All those ones from the computer's perspective, this represents the network.

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OK.

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And then the Xeros.

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Represent the host.

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So this is a network.

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This represents the host, OK?

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And the way it works is that the computer actually takes these two values and ends them together, and

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by that I mean it will take this one in this one.

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And if as long as both values are one, it will show up as a one.

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If these numbers are different or they're both zero and it's a zero, but if they're both one into one,

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so what the computer does is it takes the mask and the I.P. address ends it together and it says one

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in zero.

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OK.

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That's a zero.

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One in zero zero one in zero zero one one one one zero zero one and one one zero one one.

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Right.

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And this just goes down the entire list.

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And what you'll end up discovering when you do this is something really interesting.

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These are all zeroes, obviously, because zero 011 one.

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Two, three, four, five, six, seven, eight.

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This over here is obviously going to be zero zero zero because one in zero zero and these are all zeroes

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one zero zero zero.

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So what you end up getting are values.

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See these green values they match.

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This number here, 10 three zero.

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Why?

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Because the mask is telling you that's the network.

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This is the street that the host 34 lives on.

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These are hospitals.

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Remember, these are the hospital over here, these are the hospitals over here, and this is saying

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that this host at 34 lives on a 10 three zero network or street.

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OK, so far so good, I hope.

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Hopefully that's not too tricky.

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It can be tricky, but hopefully it's not too tricky.

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So what you can do from this is you can figure out how many hosts, you know live on this particular

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subnet.

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How do you do that?

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Let's clear off the screen so I can show you in the next lecture.

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So in the next lecture, we are going to dig into this a little deeper and I'm going to show you how

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you can see what the broadcast addresses know.

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What are the network ranges?

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How many networks do we have?

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And what IPS live on this particular network.

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OK, so this is really important, especially if you're doing port scans.

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We're trying to enumerate the network internally.

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You want to know what your boundaries are when you're crossing into a separate building and typically

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villans are aligned to networks and we'll see all this a little bit later.

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OK, so in the next lecture, we will get into this a little bit deeper.

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I will see you in the next lecture by.