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In this lesson,

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we're going to discuss

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some network port fundamentals.

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Now, when we transfer data over our networks,

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we have to find a way to tell the data

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where it's going.

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Now we can use IP addresses to get data

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to the right system, but how does it know

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what application is listening

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on that particular system?

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Well, this is where ports come into play.

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I like to think about it this way.

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If I give you my street address

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that's like my IP address on my computer.

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You'll be able to find my exact house

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within my neighborhood.

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But how are you going to find my bedroom,

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my home office, or my living room?

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Each of those rooms

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is going to serve a different function

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like sleeping, working, or relaxing.

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Now, this is where a port comes into play.

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A port is a logical opening in a computer

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that represents a service or application

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that's listening and waiting for traffic.

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So in the example of my house,

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if I give you my address

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at 123 Main Street, room number two,

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this tells you to go to the house located

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at 123 Main Street and then enter the second room,

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which is my home office

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where I'm going to be working at.

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Now, in computer networks,

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all ports are going to be numbered from zero

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all the way up to 65,535.

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Now that is a lot of ports,

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so even though you only have one IP address

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on your machine, you can have

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over 65,535 different openings for you

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to be able to run different services

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and applications on.

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Now, that large number of ports

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is actually going to be broken down

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into three different groups or categories.

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We call these the well-known ports,

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the registered ports, and the ephemeral ports.

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Now first we have well-known ports.

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These well-known ports are numbered

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from zero to 1,023.

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These ports include well-known services

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like the File Transfer Protocol,

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which exists on ports 20 and 21,

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the Simple Mail Transfer Protocol,

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which operates on Port 25,

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web browsing on port 80 and secure web browsing

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on Port 443 among many others.

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Second, we have registered ports.

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Now registered ports are numbered

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from 1,024 to 49,151.

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Now both our well-known ports

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and our reserve ports are actually going to

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be registered with an organization known

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as the Internet Assigned Numbers Authority

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or IANA pronounced iyana.

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So if you go on to create a new video game

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and you want to reserve the registered port

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of 33,333 for your game to communicate

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with other systems over,

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you're going to have to request

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that port to be reserved

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for your organization's use through IANA.

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Third, we have ephemeral ports.

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Now, ephemeral ports

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are short-lived temporary ports,

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which are open just for a short period

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of time from a predefined range of ports.

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You may also hear ephemeral ports

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called dynamic ports or private ports

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because these ports can be used by anyone

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or any system without having to request them

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like you would with a well-known

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or registered port.

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Ephemeral ports consists of any ports numbered

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from 49,152 all the way up to 65,535.

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For example, as I'm recording this lesson,

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I record the audio on a device,

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which acts as a file server

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and it's going to pick a random port somewhere

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around Port 60,000 for me to use.

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Now when I want to download the audio file

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from that device, I'm going to go to the web browser

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on my computer, and then I'll type

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in the IP address of that device as well

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as the port number that it chose.

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That way I can download that audio

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using my web browser to my main computer.

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Now, when I do this, what really happens is

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that I'm opening up a port

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on the recording device

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just for as long as I need it

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so I can download the file

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and then that port is going to close up again.

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Now, whenever we're doing data transfer,

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we're going to have to have an IP address

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and a port.

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This way we know what system we want to go to

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and what service or application

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on that system we want to communicate with.

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So what does this look like in the real world?

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Well, let's say I have a client

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and I have a website.

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The client address is going to be 192.168.1.24,

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and the website is going to be at 64.82.46.21.

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So for the client to make a request

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to this website,

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it's going to go from the source IP,

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that 192 address, and a port number,

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which is just going to be some random high port

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that we're going to pick

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from the ephemeral port range randomly

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by our system.

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Now in this case, we see port number 49,163,

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but it could have been any of the ephemeral port.

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It really doesn't matter

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because they're randomly selected.

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Now what we're going to do is take that data

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and we're going to set it to the destination

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of our website, which is going

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to be the IP address

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of 64 dot something dot something dot something

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that you see here on the web server.

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This will be our destination IP.

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Now, what port do you think it's going to send

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that data to on the web server?

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Well, if we're browsing an insecure website,

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we're going to use port 80,

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which is the default for that.

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So when the website receives the request,

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it's then going to reverse this process

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by sending data from its source IP,

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which is at 64 dot something IP address,

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and it's well-known port of Port 80.

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Then it's going to send that over

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to the destination IP, which is the IP address

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of the client, which started

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with 192 dot something dot something dot something

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and the port number that the client first used

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to initiate this communication session,

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which in this case was

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that randomly picked ephemeral port of 49,163.

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Now, at this point, we have two-way communication

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established between the web server coming

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from Port 80 and the client on port 49,163,

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and the client will send data

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from port 49,163 back to the web server

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over port 80.

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Whenever we're done transmitting data back

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and forth, then the session will be torn down

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and that random ephemeral port

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will actually be closed,

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and that way we can move on

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to the next communication session.

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Now, next time the client wants to go ahead

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and communicate the web server,

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it's still going to send the data to port 80

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because that is a well-known port,

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and the server will always be listening

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on port 80, but the client

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will now pick a new randomly generated

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ephemeral port for it to send the data from,

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and it will use that random ephemeral port

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to listen to any responses coming

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from the web server from Port 80 as well.

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So remember, when we talk about a port,

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a port is a logical opening in a computer

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that represents a service

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or application that's listening

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and waiting for communication or traffic.

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As we used in the house example,

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we have the IP address,

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which represents the building,

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but then we use the port,

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which represents the door for specific room

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inside of that building.

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It's the same thing logically

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inside of a computer, your IP address says,

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"This is the server

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or client I want to communicate with,"

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and your port number says, "This is the application

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or service I want to communicate with

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on that given server or client."