Now 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 service 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.
Okay.
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
topology.
I'll select a traditional PC and add it to the topology.
Now 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.
So 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.
So in this application I simply select a change symbol and then I could specify a nother type of symbol
to represent a device.
I 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 service.
The point is 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.
So 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.
So what I'm going to 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.
Now today we don't have to do that.
I'll discuss order MDI X late in the course.
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.
So once again I'll delete that cable and I'll select a cross over cable copper crossover cable and connect
fast ethernet on the PC to fast ethernet on the server.
So there you go.
I've built a network.
That's an example of a network.
It's a logical representation of a physical network.
OK.
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 pre-built 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 have been duplicate MAC addresses but in general that isn't a problem.
Every device has its own MAC address.
So if I look at this PC and go to config go to foster Ethernet0.
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 center.
Have a look at your Adapter Settings.
So as an example this Wi-Fi 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 Wi-Fi 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
first Ethernet0 notice she has the MAC address of the server but what I'll do is change that so
I'll change the MAC address.
This is a twelve digit number in hexadecimal.
Now I'll discuss hexadecimal and the details of this later in the course but for the moment just notice
there are 12 numbers there hexadecimal numbers.
So 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 0 1 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 PC using
a protocol called Dynamic Host Configuration Protocol or DHCP
So DHCP is allocating IP addresses to your PC, I can see that here, by opening up a command
prompt and if I type IP config an IP address in this case IP version 4 is configured on my Windows laptop
so those 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.
So 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 of using Packet Tracer.
I'm configuring the IP addresses manually, Okay so if I go a desktop and open up a command prompt I'll
make this a bit bigger and top the command IP config.
What you'll notice is this PC has an IP address.
I'll do something similar on the server.
Let's give the server IP address of 10.1.1.2 and a subnet mask of 255.255
0.
So 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 servers replying back "YES I'M HERE" and the PC saying "HELLO ARE YOU THERE?"
and the server replies back "YES I'M HERE!"'
So basically it sends the he sends a message to the server saying Please reply if you're there and the
server replies back saying, Yes I'm here and it's used just to verify that the server is up, is it there
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
there we are getting a reply from 10 1 1 2 the PCs, IP addresses this, the servers IP address
use the command IP config to see it make this bigger is 10 1 1 2 Okay 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 reply back saying, Yes I'm here.
So this server, is there but that doesn't really help us what we want to 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.
So on the HTTP Service, I'm going to leave this on that's the default on the PC if I close this command
prompt down and open up a web browser and deletes browse using HTTP Hypertext Transfer Protocol to the
IP address of the server.
What you'll notice is a web page displays I can see a small Web page here which says hello world.
I can go back, I can go look at an image page and here's a Cisco logo.
Now 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 web page to the client when the client
requested the web page.
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
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 10.1.1.1.2
Now what will happen and it might take it a while, is it will time out and there you go.
Notice it says request timed out 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.
So if you send it a request saying show me a web page on port 80.
It just basically drops the request, ignores what you, what you're asking.
If I go HTTPS, however, notice we get to the web page 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.
So just to prove it because that wasn't very clear I'll open up a web browser again and manually type
HTTP 10.1.1.2 and notice the web page displays.
Now another great thing about packet tracer is we've got the simulation mode.
If I select a simulation mode I can actually see what's going on.
So 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.
Now 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 TCPIP 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.
Wireshark
will 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 PC
destination IP addresses is the server, we're using IP version 4 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 you sent something to a server and it gets lost it'll be re-sent, retransmitted.
So it gives us reliability.
Destination Port number is 80.
So when we send that into the network that will be sent to the server and the server will receive that.
And then what it will do is send something back to the PC
so a packet gets sent back from the server to the P.C. 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.
Now 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, etc 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, so build this yourself and try it yourself.