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Now analogies are okay but it's much better to demonstrate this practically using Packet Tracer.

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So let's actually build a network together and I'll show you how clients and service can work together

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and I'll show you that a client can also become a server.

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You could have two laptops one providing a service to another laptop so the laptop can be a server in

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some cases or it can be a client.

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In other cases.

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Okay.

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So in packet trace I'm going to go to end devices and I'm going to select a server and add it to my

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topology.

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I'll select a traditional PC and add it to the topology.

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Now these are symbols or representations of devices physically.

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This is a server physically that's a laptop.

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But logically that's what they look like.

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These are once again just symbols in an application like Packet Tracer in networking we use network

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diagrams to explain what a network looks like.

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Another example would be here where I'm using an application called GNS3.

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So here's a Windows Server and here's a PC.

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Notice different symbols for different types of devices.

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I could change the symbol if I wanted to.

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So in this application I simply select a change symbol and then I could specify a nother type of symbol

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to represent a device.

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I notice there are many types of devices shown here but as an example I could say this is a server cluster.

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If it was a cluster of service.

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The point is is that a symbol is simply a logical representation for a physical device in Packet Tracer.

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If I click on this server there's a physical representation of the server and then I could once again

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change the symbol or icon used to represent that server

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here's the physical representation of the PC notice.

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Ethernet port right over there.

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So again this is a logical representation of a device.

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Now these two devices can communicate with each other.

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We either need to communicate using a physical cable or we need to communicate using the air.

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So what I'm going to do is select connections and I'll select what's called a crossover cable, a crossover

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cable allows two PCs to talk directly to each other in the old days we had to use what was called

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a crossover cable to allow PC to talk to PC.

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Now today we don't have to do that.

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I'll discuss order MDI X late in the course.

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This used to be a problem in the old days it's not such a problem these days notice the interfaces have

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gone green but in packet tracer if I delete that cable and then use just a standard ethernet cable

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notice the interface is down because Packet Tracer still expects you to use the right cable.

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So once again I'll delete that cable and I'll select a cross over cable copper crossover cable and connect

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fast ethernet on the PC to fast ethernet on the server.

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So there you go.

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I've built a network.

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That's an example of a network.

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It's a logical representation of a physical network.

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OK.

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But to allow these two devices to communicate with each other we need to have two things we need to

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have an ethernet address or MAC address that's pre-built into network interface cards

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So a manufacturer will burn a MAC address onto a network interface card.

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You can change MAC addresses but generally you don't have to.

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They are globally unique.

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There have been cases where there have been duplicate MAC addresses but in general that isn't a problem.

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Every device has its own MAC address.

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So if I look at this PC and go to config go to foster Ethernet0.

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Here is the MAC address of the PC, you can see your mac address on a Windows computer as an example

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by going to control panel, network and Internet network and sharing center.

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Have a look at your Adapter Settings.

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So as an example this Wi-Fi network adapter has this MAC address.

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That's its physical address or MAC address.

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You'll see something similar on an iPhone or other devices.

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Every device that communicates on physical Ethernet or communicates on Wi-Fi is gonna have a mac address

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we don't have to configure that again that's configured by default by the manufacturer on the server

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first Ethernet0 notice she has the MAC address of the server but what I'll do is change that so

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I'll change the MAC address.

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This is a twelve digit number in hexadecimal.

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Now I'll discuss hexadecimal and the details of this later in the course but for the moment just notice

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there are 12 numbers there hexadecimal numbers.

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So that's the MAC address now of the server.

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On the PC I'll change the MAC address as well and I'll make this a bunch of ones so triple 0 1 followed

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by eight ones.

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Now again you don't have to change the MAC address.

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I'm just doing that to make things simple now the second thing we need is an IP address typically in

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networks a server which could be a home router we'll be allocating IP addresses to your PC using

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a protocol called Dynamic Host Configuration Protocol or DHCP

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So DHCP is allocating IP addresses to your PC,  I can see that here, by opening up a command

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prompt and if I type IP config an IP address in this case IP version 4 is configured on my Windows laptop

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so those windows laptop has received an IP address automatically from a DHCP server or it could be configured

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statically you can configure IP addresses statically in our little network.

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We don't have a router or another type of device so we have to manually configure the IP addresses.

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So on this first PC. I'm gonna give it an IP address of 10.1.1.1  and a subnet mask

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of 255.255.255.0

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Don't worry too much about IP addresses for the moment just understand that we've configured a MAC address

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you don't have to do that and I've configured an IP address.

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You don't have to do that if you've got two Windows laptops they can automatically communicate with

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one another here because of using Packet Tracer.

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I'm configuring the IP addresses manually, Okay so if I go a desktop and open up a command prompt I'll

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make this a bit bigger and top the command IP config.

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What you'll notice is this PC has an IP address.

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I'll do something similar on the server.

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Let's give the server IP address of 10.1.1.2 and a subnet mask of 255.255

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0.

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So on the PC now I can use a special application called Ping to verify connectivity to the server.

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Ping is basically sending a message saying "HELLO ARE YOU THERE?"

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And then the servers replying back "YES I'M HERE" and the PC saying "HELLO ARE YOU THERE?"

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and the server replies back "YES I'M HERE!"'

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So basically it sends the he sends a message to the server saying Please reply if you're there and the

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server replies back saying, Yes I'm here and it's used just to verify that the server is up, is it there

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is it working, is it turned on, so on the PC, I can use the command ping 10.1.1.2 and as you can see

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there we are getting a reply from 10 1 1 2 the PCs, IP addresses this, the servers IP address

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use the command IP config to see it make this bigger is 10 1 1 2 Okay but that doesn't really prove

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anything except that my PC can ping the server that I've got IP connectivity to the server I basically

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sent a message to the server saying, Are you there and the server reply back saying, Yes I'm here.

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So this server, is there but that doesn't really help us what we want to do is have a look at the services

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running on the server and notice here are a whole bunch of services we've got HTTP as an example we've

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got DNS which is domain name system basically allows me to resolve domain names such as Google.com

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or Facebook.com to an IP address, we've got other types of services here like email, FTP and various

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other services.

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So on the HTTP Service, I'm going to leave this on that's the default on the PC if I close this command

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prompt down and open up a web browser and deletes browse using HTTP Hypertext Transfer Protocol to the

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IP address of the server.

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What you'll notice is a web page displays I can see a small Web page here which says hello world.

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I can go back, I can go look at an image page and here's a Cisco logo.

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Now that doesn't really do much but that's an example of a network.

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We've got a client on the left a server on the right.

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The server is configured with the HTTP Service and it's serving a web page to the client when the client

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requested the web page.

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If I turn the service off so I've turned it off now for HTTP but it's on for HTTPS, HTTP has once again

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just an encrypted version of hypertext transfer protocol which is used for web browsing.

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I'll close this down open it up again try and go to HTTP 10.1.1.1.2

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Now what will happen and it might take it a while, is it will time out and there you go.

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Notice it says request timed out and that's because the server is no longer listening on port 80.

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It's not listening.

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So it just basically drops the traffic that you send to it.

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So if you send it a request saying show me a web page on port 80.

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It just basically drops the request, ignores what you, what you're asking.

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If I go HTTPS, however, notice we get to the web page because the server is listening on Port 443 now

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which is the port number used for HTTPS, I'll enable HTTP once again.

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And let's try and go to the server on port 80 and that's worked.

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So just to prove it because that wasn't very clear I'll open up a web browser again and manually type

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HTTP 10.1.1.2 and notice the web page displays.

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Now another great thing about packet tracer is we've got the simulation mode.

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If I select a simulation mode I can actually see what's going on.

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So on the PC and I'll just move this around so we can see what's going on.

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I'm gonna click Go now and what you'll notice is a packet has been sent into the network.

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Now there's a lot of information here and I'll go through this in more detail when we talk about the

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OSI model and the TCPIP model.

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But what I'd like you to see is we've got a MAC address which is the MAC address of the PC sending

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traffic to the MAC address of the server and we've got an IP address which is the IP address of the

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PC going to the IP address of the server, source IP address is PC, destination IP address is the server

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at layer 2 here we've got an Ethernet Header, source MAC address is the PC, destination MAC address

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is the server.

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Notice here we've got port 80, port 80 once again is the port number that the server is listening on.

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If we look at that in more detail and don't get scared by this.

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This is just an example of the kind of data that's sent into the network.

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Wireshark

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will show you this in more detail than this but this gives you an idea of what's going on.

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Source MAC address is the, PC destination MAC address is the server, we've got source IP address PC

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destination IP addresses is the server, we're using IP version 4 here, we're also using a protocol called

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TCP or transfer control protocol that basically just gives us reliability in a network transmission.

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So if you sent something to a server and it gets lost it'll be re-sent, retransmitted.

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So it gives us reliability.

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Destination Port number is 80.

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So when we send that into the network that will be sent to the server and the server will receive that.

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And then what it will do is send something back to the PC

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so a packet gets sent back from the server to the P.C. with information.

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Notice the source port for the reverse traffic from the server to the PC is 80.

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Basically, the client talks to the server on port 80 and then the server replies back from port 80 to a port

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number that your client has randomly decided to use.

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Now that's a lot of information and I'm hoping it's not too overwhelming the best thing you can do is

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install packet tracer and build a topology yourself and follow what I'm doing.

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There's no better way to learn than to do things yourself.

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One of my favorite analogies is riding a bicycle.

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I can talk about riding a bicycle.

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I can show you pictures, videos, etc but until you yourself ride a bicycle fall off a few times struggle

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a bit you'll never learn to ride a bicycle.

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Best way to learn is to just do it, so build this yourself and try it yourself.