- Poodle, what's that?
- I think it's something like a router.
- It's a router?
I think it's a server.
- A server? What's a server?
- A server, we're going to learn about servers
in this video.
What's that over there behind us?
- A laptop.
- A laptop. Which one do you use?
- A laptop rather.
- You use a laptop rather than a server.
In a previous video, which I've linked here on below,
I explained a little bit about servers and clients,
but Poodle don't you think it will be better
if we practically demonstrate what those are in a network
and to do that, we're gonna use Cisco Packet Tracer.
Is that good?
- Yes.
- Okay, let's do that then.
(bright upbeat music)
Analogies are okay, but it's much better
to demonstrate this practically using Packet Tracer.
So let's actually build a network together and I'll show you
how clients and servers can work together,
and I'll show you that a client can also become a server.
You could have two laptops,
one providing a service to another laptop,
so the laptop can be a server in some cases,
or it can be a client in other cases.
In Packet Tracer, I'm gonna go to End Devices
and I'm going to select a server and add it to my topology.
I'll select a traditional PC and add it to the topology.
These are symbols or representations of devices.
Physically, this is a server.
Physically, that's a laptop,
but logically that's what they look like.
These are once again, just symbols
in an application like Packet Tracer.
In networking, we use network diagrams
to explain what a network looks like.
Another example would be here
where I'm using an application called GNS3.
Here's a Windows server and here's a PC.
Notice different symbols for different types of devices.
I could change the symbol if I wanted to.
In this application, I simply select "Change symbol"
and then I could specify another type of symbol
to represent a device.
And notice there are many types of devices shown here,
but as an example, I could say, this is a server cluster,
if it was a cluster of servers.
The point is that a symbol is simply
a logical representation for a physical device.
In Packet Tracer, if I click on this server,
there's a physical representation of the server,
and then I could once again change the symbol or icon used
to represent that server.
Here's the physical representation of the PC.
Notice Ethernet port right over there.
Again, this is a logical representation of a device.
Now these two devices can communicate with each other.
We either need to communicate using a physical cable
or we need to communicate using the air.
What I'm gonna do is select connections
and I'll select what's called a crossover cable.
A crossover cable allows two PCs
to talk directly to each other.
In the old days,
we had to use what was called a crossover cable
to allow PC to talk to PC.
Today, we don't have to do that.
I'll talk later about Auto MDI-X.
This used to be a problem in the old days.
It's not such a problem these days.
Notice the interfaces have gone green,
but in Packet Tracer if I delete that cable
and then use just a standard Ethernet cable,
notice the interface is down because Packet Tracer
still expects you to use the right cable.
Once again, I'll delete that cable
and I'll select a copper crossover cable,
and connect to FastEthernet on the PC
to FastEthernet on the server.
There you go.
I've built a network.
That's an example of a network.
It's a logical representation of a physical network.
But to allow these two devices
to communicate with each other, we need to have two things.
We need to have an Ethernet address or Mac address.
That's prebuilt into network interface cards.
So a manufacturer will burn a Mac address
onto a network interface card.
You can change Mac addresses,
but generally you don't have to.
They are globally unique.
There have been cases
where there've been duplicate Mac addresses,
but in general, that isn't a problem.
Every device has its own Mac address.
If I look at this PC and go to Config,
go to FastEthernet0,
here is the Mac address of the PC.
You can see your Mac address on a Windows computer
as an example, by going to Control Panel,
Network and Internet,
Network and Sharing Centre,
have a look at your adapter settings.
So as an example, this WiFi network adapter
has this Mac address.
That's its physical address or Mac address.
You'll see something similar on an iPhone or other devices.
Every device that communicates on physical Ethernet
or communicates on WiFi is gonna have a Mac address.
We don't have to configure that again.
That's configured by default by the manufacturer.
On the server, FastEthernet0,
notice here is the Mac address of the server.
But what I'll do is change that.
I'll change the Mac address.
This is a 12 digit number in hexadecimal.
I'll discuss hexadecimal
and the details of this later in the course,
but for the moment,
just notice they are 12 numbers there, hexadecimal numbers.
That's the Mac address now of the server.
On the PC, I'll change the Mac address as well,
and I'll make this a bunch of ones.
So triple zero, one followed by eight ones.
Now again, you don't have to change the Mac address.
I'm just doing that to make things simple.
Now, the second thing we need is an IP address.
Typically in networks, a server,
which could be a home router,
we'll be allocating IP addresses to your PCs,
using a protocol
called Dynamic Host Configuration Protocol or DHCP.
DHCP is allocating IP addresses to your PC.
I can see that here by opening up a command prompt,
and if I type ipconfig an IP address,
in this case, IPv4 is configured on my Windows laptop.
This Windows laptop has received an IP address
automatically from a DHCP server,
or it could be configured statically.
You can configure IP addresses statically.
In our little network, we don't have a router
or another type of device,
so we have to manually configure the IP addresses.
On this first PC,
I'm gonna give it an IP address of 10.1.1.1
and a subnet mask of 255.255.255.0.
Don't worry too much about IP addresses.
For the moment, just understand
that we've configured a Mac address,
you don't have to do that and I've configured an IP address.
You don't have to do that if you've got two Windows laptops,
they can automatically communicate with one another here
because using Packet Tracer,
I'm configuring the IP addresses manually.
If I go to Desktop and open up a command prompt,
I'll make this a bit bigger,
and type the command ipconfig,
what you'll notice is this PC has an IP address.
I'll do something similar on the server.
Let's give the server an IP address of 10.1.1.2
and a subnet mask of 255.255.255.0.
On the PC now I can use a special application
called Ping to verify connectivity to the server.
Ping is basically sending a message saying,
"Hello, are you there?"
And then the server is replying back,
"Yes, I'm here."
And the PC is saying, "Hello, are you there?"
And the server replies back, "Yes, I'm here."
Basically, the PC sends a message to the server saying,
"Please reply if you're there."
And the server replies back saying, "Yes, I'm here."
It's used just to verify that the server is up.
Is it there?
Is it working?
Is it turned on?
On the PC I can use the command ping 10.1.1.2
and as you can see there,
we are getting a reply from 10.1.1.2.
The PCs IP address is this.
The server's IP address,
use the command ipconfig to see it,
make this bigger, is 10.1.1.2.
But that doesn't really prove anything
except that my PC can ping the server,
that I've got IP connectivity to the server.
I basically sent a message to the server saying,
"Are you there?"
And the server replied back saying, "Yes, I'm here."
This server is there, but that doesn't really help us.
What we wanna do is have a look at the services
running on the server,
and notice here are a whole bunch of services.
We've got HTTP, as an example,
we've got DNS, which is Domain Name System,
basically allows me to resolve domain names,
such as google.com or facebook.com to an IP address.
We've got other types of services here, like EMAIL, FTP,
and various other services.
On the HTTP service,
I'm gonna leave this as on.
That's the default.
On the PC, if I close this command prompt down
and open up a web browser
and let's browse using HTTP Hypertext Transfer Protocol
to the IP address of the server.
What you'll notice is a webpage displays.
I can see a small webpage here, which says, "Hello, world!"
I can go back.
I can go and look at an image page,
and here's a Cisco logo.
That doesn't really do much,
but that's an example of a network.
We've got a client on the left, a server on the right.
The server is configured with the HTTP service
and it's serving a webpage to the client
when the client requested the webpage.
If I turn the service off,
I've turned it off now for HTTP,
but it's on for HTTPS.
HTTPS is once again, just an encrypted version
of Hypertext Transfer Protocol,
which is used for web browsing.
I'll close this down, open it up again,
try and go to http://10.1.1.2.
What will happen and it might take it a while,
is it will timeout.
And there you go.
Notice it says "Request Timeout"
and that's because the server
is no longer listening on port 80.
It's not listening so it just basically drops the traffic
that you send to it.
If you send it a request saying,
"Show me a webpage on port 80"
it just basically drops the request,
ignores what you're asking.
If I go HTTPS however,
notice we get to the webpage
because the server is listening on port 443 now,
which is the port number used for HTTPS.
I'll enable HTTP once again,
and let's try and go to the server on port 80
and that's worked.
Just to prove it, cause that wasn't very clear,
I'll open up a web browser again
and manually type http://10.1.1.2
and notice the webpage displays.
Another great thing about Packet Tracer
is we've got the simulation mode.
If I select to simulation mode,
I can actually see what's going on.
On the PC, and I'll just move this around
so we can see what's going on.
I'm gonna click Go now,
and what you'll notice is a packet
has been sent into the network.
There's a lot of information here
and I'll go through this in more detail
when we talk about the OSI model and the TCP/IP model.
But what I'd like you to see is we've got a Mac address,
which is the Mac address of the PC,
sending traffic to the Mac address of the server,
and we've got an IP address,
which is the IP address of the PC
going to the IP address of the server.
Source IP address is PC, destination IP address
is the server at Layer 2.
Here we've got an Ethernet Header.
Source Mac address is the PC,
destination Mac address is the server.
Notice here we've got port 80.
Port 80 once again is the port number
that the server is listening on.
If we look at that in more detail
and don't get scared by this.
This is just an example of the kind of data
that's sent into the network.
Why shock? We'll show you this in more detail than this,
but this gives you an idea of what's going on.
Source Mac address is the PC,
destination Mac address is the server.
We've got source IP address is PC,
destination IP address is the server.
We're using IPv4 here.
We're also using a protocol called TCP
or Transfer Control Protocol.
That basically just gives us reliability
in a network transmission.
So if we sent something to a server and it gets lost,
it will be resent, retransmitted.
It gives us reliability.
Destination port number is 80.
When we send that into the network,
that will be sent to the server
and the server will receive that,
and then what it'll do is send something back to the PC.
A packet gets sent back from the server
to the PC with information.
Notice the source port for the reverse traffic
from the server to the PC is 80.
Basically, the client talks to the server on port 80,
and then the server replies back from port 80
to a port number that your client
has randomly decided to use.
That's a lot of information
and I'm hoping it's not too overwhelming.
The best thing you can do is install Packet Tracer
and build a topology yourself,
and follow what I'm doing.
There's no better way to learn than to do things yourself.
One of my favorite analogies is riding a bicycle.
I can talk about riding a bicycle,
I can show you pictures, videos, et cetera,
but until you, yourself ride a bicycle,
fall off a few times, struggle a bit,
you'll never learn to ride a bicycle.
Best way to learn is to just do it.
Build this yourself and try it yourself.
(bright upbeat music)