WEBVTT

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 Hello and welcome to this video titled,
 Why Do We Need IPv4 Addresses?

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 In this video I'm going to talk about
 an introduction to IPv4 addressing,

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 so you'll learn the structure
 of an address and what it is.

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 We'll break it down into network and
 host bits and understand why it's

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 important to know the difference
 between those two.

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 And we'll understand packet routing
 decisions based on the ability to

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 look at the network and host
 bits of an IP address.

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 Alright, so let's start by talking
 about IPv4 addressing.

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 So if you've studied IPv4 and you know
 anything about the IPv4 header

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 that's placed in front of your data,
 you'll know that probably one of

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 the most critical components of that IPv4
 header is the source and destination

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 address. After all, that's the main
 reason why we use IP in the first

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 place is to get packets between remote networks
 from a source to a destination.

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 So let's focus in now on just those
 addressing fields leaving behind all

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 the other fields of
 the IPv4 header.

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 So the IPv4 address
 is 32 bits wide.

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 So that's how many bits
 it is end to end.

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 So the source address is 32 bits.

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 Destination address is 32 bits.

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 So right here, this is an example of
 how your laptop, PC or smartphone

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 sees its IP address.

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 Of course, it sees it
 as a binary number.

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 We don't talk about it as binary numbers,
 but this is how computing devices

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 view their IP address.

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 And that is right there, 32 bits
 if you count them end to end.

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 Now an IP address is
 a logical address.

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 What does that mean?

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 That means that something about this
 address indicates geographically

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 where your system is right now.

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 Now geographical might mean it indicates
 somewhere in that address the

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 department you're a part of, the floor
 that you're on, the room that you're

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 in, maybe the section of the campus
 that you're working from right now.

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 And so because of that, if you move
 your device around like a laptop or

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 a tablet so frequently does, if you
 move from room to room or department

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 to department or from building to building,
 your IP address will change.

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 Because an overall large computer network
 is subdivided into different

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 sections and each section has
 a different IP network.

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 So part of this address is going to be
 the network portion of the address.

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 Your laptop, your PC, when it sees this
 32-bit number has to be able to

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 identify which of those bits represent
 the network that it's on right

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 now. And if it moves to a different network,
 for example, if you disconnect

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 from the WiFi access point that you're
 on right now and you move your

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 laptop over and you connect to a new
 WiFi access point in a different

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 department, your networking
 bits will change.

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 Well actually your entire 32-bit address
 will change, but the part that's

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 most critical to your laptop will be
 representing this on a new network.

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 So this 32-bit address is subdividable
 into two different components.

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 Left to right, some of the bits from
 left to right will be the network

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 portion, for example,
 maybe this right here.

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 Maybe these bits right here are your
 networking bits and these bits right

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 here are your host bits.

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 And it's very critical for any device running
 IP, whether it be a smartphone,

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 laptop, tablet, whatever, to be able
 to find where that dividing line

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 is between the network
 bits and the host bits.

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 So this pattern, so theoretically,
 this is just theoretical right now,

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 if this was where my network bits right
 here, that would mean that if

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 I'm constructing an IP packet and the
 destination IP address of that packet

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 had this exact same pattern, that would
 mean that destination resides

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 in the same part of the
 network where I am.

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 Whereas if the destination bits is
 some different pattern, that would

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 mean, logically speaking, that the
 destination I'm trying to reach is

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 not in my section of the network, but
 a different section of the network,

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 which would mean my packet would
 have to go through a router.

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 The only network my laptop knows about
 is a network he's sitting on right

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 now, which is this as an example.

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 Any other bit pattern is like telling
 my laptop, hey, go to Jupiter, go

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 to Mars, go to the Nebula Galaxy.

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 My laptop has no idea where the heck
 those things are, but what it does

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 know is that in my network, in this
 network, there's a device with an

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 IP address that starts out with the
 same pattern who is a router, who

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 is a gateway, and if I send my packet
 to him, he knows how to get it to

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 those remote destinations.

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 Now, we as human beings can't deal with
 IP addresses, they're in binary

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 like that. No problem for a computing
 device, but we can't work with that.

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 So we sort of reformat this address,
 we look at it in a different way

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 so it's something that's human
 readable that we can handle.

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 So here's what we do.

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 Step number one is we take that 32
-bit pattern there and we divide it

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 up into four octets.

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 Okay, remember an octet is
 a grouping of eight bits.

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 So here I've just divided it into four
 equal pieces, each one is called

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 an octet or a byte, and then we say,
 okay, we convert each one of those

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 bytes from a binary number into a decimal
 number, and then we put a dot

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 between those decimal numbers to represent
 that each one of those decimal

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 numbers represents eight
 bits of binary.

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 So you can see here the 192,
 that's my first eight bits.

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 If you know anything about binary,
 you could add that up.

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 You know the 128 and the 64 bits
 are turned on, giving us 192.

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 The next grouping of eight bits translates
 to the number one in decimal.

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 The next group is the number one in
 decimal and then the final octet in

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 binary translates to
 three in decimal.

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 So this is called dotted decimal notation,
 and this is how you will see

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 and manipulate IP addresses is
 using dotted decimal notation.

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 But just be aware that whenever you
 go into any system, whether it be

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 your laptop, your PC, a router, a switch,
 and you type in an IP address,

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 even though you're typing it as dotted
 decimal in the background, that

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 computing device is translating it
 into one big long string of 32 bits

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 in binary. Okay, so as I mentioned, devices
 that have IP before addresses,

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 whatever they are, whether it be your wireless
 thermostat or your refrigerator,

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 has to be able to know
 what network it's on.

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 It has to be able to differentiate the
 networking bits from the host bits.

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 Once it's able to do that, that will
 help it determine, okay, now that

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 I've created an IP packet and I'm going
 to do, I'm going to compare the

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 destination with the
 source, with myself.

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 So anytime a laptop PC smartphone creates
 an IP packet, and it might be

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 creating thousands of IP packets every
 second, every time it creates one,

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 it compares the destination IP address
 with its own source IP address.

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 And that device says, okay, well, of
 this 32-bit number, which represents

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 me, I know these first few bits
 are my networking bits.

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 That's the group or the part
 of the network I'm in.

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 Let me look at the destination.

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 Are the same bits in the destination
 matching my group?

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 In other words, are the networking bits
 I have the same as the networking

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 bits in this destination?

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 If the answer is no, then it says, okay,
 this destination is off network.

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 It's not on my network, it's
 on some other network.

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 I need to forward this packet to my
 router, a default gateway who is in

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 my network who can
 get it out there.

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 If the answer is yes, that means, oh, okay,
 I can speak to that guy directly.

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 I don't need to get
 the router involved.

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 An analogy I sometimes make
 is think of it like this.

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 Imagine that you are standing in
 the middle of your street, okay?

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 And I tell you, hey, I want you to, let's
 say your street is first street,

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 and I say, hey, I want you to
 talk to Bob on first street.

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 Well, first street is your network, and
 you say, oh, Bob on first street.

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 Well, I live on first street.

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 He lives on first street.

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 Therefore, I can just shout,
 hey, Bob, are you out there?

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 And if Bob's on first street like me,
 he should be able to hear me shouting

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 to him, and he and I should
 be able to talk directly.

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 But if I say, hey, I need you to
 talk to Sally on 17th street.

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 Well, now you say, oh, okay, well, someone
 called Sally, great, but she's

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 on 17th street. That's
 not my street.

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 You can shout as long as you want until
 the cows come home, and Sally's

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 not going to hear you because
 she's not on your street.

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 She could be in a different
 country for all you know.

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 So in that case, where
 would you go?

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 You would turn to the
 postal service.

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 That's analogous to your router.

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 And you'd say, hey,
 postal service.

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 If I give you my message for Sally on
 17th street, can you get it there?

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 And if the postal service knows what
 they're doing, they'll say, of course

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 we can. So you give your
 message to them.

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 That's like giving your
 data to your router.

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 And then they somehow behind the scenes,
 you don't know how they do it.

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 You don't care how they do it.

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 They get that information
 to Sally on 17th street.

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 That's sort of like how routing
 happens with these packets.

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 So here's an example, a more realistic
 or practical example here.

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 So notice that we have Bob and Sally,
 and they have been able to identify

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 of their own unique 32-bit addresses that
 the red bits are their networking

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 bits. Now in this video, we're not
 going to go into how do they figure

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 that out? Of this 32-bit number, how
 do I figure out which bits are my

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 networking bits?

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 We have another video coming up
 that will cover that topic.

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 But let's assume that logic
 has already been done.

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 And so Bob and Sally recognize that these
 bits here in red are their networking

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 bits. So everybody on their network
 has that same pattern.

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 Now Bob and Sally happen
 to be in the library.

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 So everybody in the library, when they
 get an IP address, the first few

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 bits is going to match
 this common pattern.

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 That's their networking bits.

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 Now down below though,
 that's not the library.

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 Down below, that's the
 biology department.

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 So because that's a completely different
 network, their networking bits

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 have to take a different pattern.

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 Now if Bob wants to send a packet to
 Sally, he's going to create that

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 packet and he's very quickly going
 to recognize by doing a comparison

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 of his source address against the destination
 that Sally's address is

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 in the exact same network
 as himself.

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 So he can just use this switch
 right here to talk to her.

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 They can just talk directly back
 and forth with that switch.

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 Similarly, if Sam wants to talk to
 Sue, they're in the same network.

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 They can just use this switch.

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 But if Sam wants to talk to Sally or Sam
 wants to talk to Bob, very quickly

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 going to recognize that okay, Sam
 says this is my networking bits.

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 The bits I'm trying
 to get to is this.

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 Hmm, not the same pattern,
 different pattern.

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 So now Sam is going to have to redirect
 his information to a router.

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 And the router is going to have to somehow
 learn that this pattern right

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 here is reachable via
 interface number one.

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 And that this pattern right here is
 reachable via interface number two.

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 Once the router knows that information,
 it can route packets back and

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 forth. Between these two networks.


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 So that concludes this video.

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 Thank you for watching.