OK so in packet tracer I'm gonna start simulation mode on my windows P.C. I'm gonna connect via HTTP to
the server on the left which is 10 1 1 100.
Notice nothing happens but when we go back here we can see that some packets have been created.
First one is ARP now on ethernet devices communicate using what's called a MAC address and MAC addresses
are burnt in address on a network interface code.
So I'm connecting from this P.C. to the server.
The P.C. doesn't know the MAC address of the server.
This is the MAC address of the P.C. and we can see that by going to the config of the P.C. go to Fast
Ethernet.
Notice MAC address ends in 11 99.
So this PC is basically arping or requesting, ARP is a resolution protocol requesting the MAC address
of the server.
So it's basically saying who has this IP address so that gets sent to the switch.
Now this is what's called a broadcast frame.
Notice the destination is Fs.
That's a broadcast basically saying who has this IP address.
If we look at the PDU or protocol data unit what you'll notice is the target MAC address is blank
target IP address is this.
Now this is a broadcast and a layer 2 switch will flood the broadcast which basically says it sends
it out of all ports so it gets sent to this server this server drops the message because it doesn't have
the IP address requested by the P.C..
This router will also drop the packet but this server will reply back.
So the inbound PDU is from the P.C. to a broadcast address but the reply is now from the server
ending in 0 0 8 6 as its MAC address to the P.C..
Notice how Packet Tracer uses 7 layers here.
At the moment it only shows Layer 1 and 2 here.
If we look at the inbound PDU however at layer 2 we have Ethernet at layer 3 we have ARP and
in the outbound PDU that's what we see something similar.
Layer 2, Layer 3, ARP, target IP address is the P.C. source IP address is the server source MAC address
is the server and we can see that by looking at the interface notice 0 0 86 is the MAC address.
So what happens now is that gets sent back to the switch and gets sent to the P.C..
So now the P.C. knows the MAC address of the server and can communicate directly to the server.
notice this is a TCP packet, so in TCP before communication takes place they do what's called a three
way handshake.
They agree on certain parameters such as sequence numbers and how much data they can send and then an
HTTP packet is sent into the network.
So if we look at the actual HTTP and that's the one I want to concentrate on here.
Let's look at the HTTP packet or HTTP message. Packet Tracer shows this very nicely gigabit 1 0 form
the switch receives the frame.
So this interface receives the frame from the P.C. we've got Layer 1 and layer 2 information here.
It's gonna be forwarded out of gigabit 1 0 2 so forwarded to the server.
But let's look at the inbound PDU and outbound PDU or protocol data unit in a lot of detail.
Source MAC address is the P.C. destination MAC address is the server.
We have a type field.
How does one layer referred to the layer above it at layer 2 on Ethernet
It uses a type field this type 0 800 in hexadecimal 0x means hexadecimal indicates that the higher
layer protocol is IP version 4.
We could have other protocols here like IP version 6.
So if I go back to a ARP packet and have a look at that.
Notice the type field is different here.
It's 0 8 0 6.
That indicates ARP.
So when a device receives a frame at layer 2 it needs to know which protocol to use.
In other words which protocol stack to use.
We can see that in Windows by going to control panel.
And if we have a look at our adapter so this is the wireless adapter that I'm currently using and go
to properties.
What you'll notice is we have IP version 4 and IP version 6, 2 protocols at layer 3 have been enabled
in this P.C. when the P.C. receives frames of the wire.
So when that P.C. receives data how does it know which protocol stack to use.
In other words is it an IP version 4 packet or is it an IP version 6 packet.
It's based on the type field at layer 2.
So this server on the left here when it receives this HTTP packet will know that it needs to use the IP
version 4 protocol stack.
If you send IP version 4 packets to IP version 6 it's not going to understand it in the same way that
I have an English protocol stack and I have an Afrikaans protocol stack. I'm much better in English than
I am at Afrikaans if I speak to you and say goeie more Hoe gaan dit
If you don't understand Afrikaans you're not going to understand what I'm talking about.
If I say good morning how are you and use the English protocol stack that's gonna make a lot of sense
if you speak multiple languages and you hear different languages your mind will just switch to that
language very easily.
But on a P.C. it needs to know which protocol stack or which language to use.
So in this example IP version 4 is the layer 3 protocol.
And here we can see IP version 4 source and destination ip addresses now.
Same thing again.
How does it know which protocol is being used at layer 4. Ethernet is the layer 2 encapsulation IP version
4 is the layer 3 protocol TCP is the layer 4 protocol.
This number here indicates so it's a 6 in hexadecimal or 6 in decimal indicates that the protocol used
at layer 4 is TCP.
You can find protocol information easily by just searching for IP version 4 protocol numbers and then
the IANA website gives you a list of protocol numbers.
So as an example TCP is protocol number 6 17 if we scroll down is a UDP TCP or transmission control
protocol is reliable.
User datagram protocol or UDP is not reliable.