So guys going forward
Now so, let us do summarization of what we have discussed in this module so far
So if receiving machine receives a packet in which destination IP address
Exactly matches with the IP address of one of its local interface,
Receiving machine consumes the packet so that we have already discussed that if the destination IP address that is specified
in the IP header of the packet
exactly matches to the IP address of one of the local interface of
Receiving machine then that particular machine do not forwards a packet, It just consumes the packet
Why? because if the IP address is specified in the packet
Matches exactly with the IP address configured on one of the local interface of the machine
It means that the packet is destined to that particular machine only
Second point is that if receiving machine receives a packet in which destination IP address do not
Matches exactly with the IP address of one of its local interface
but
falls in one of its local subnet in
This particular case the Machine forwards the packet on to that subnet using l2 routing
Right, so we will discuss more on this l2 routing when we will be discussing l2 routing in detail
So you understood this point that whenever a packet is received
Such that the destination IP address is specified in the packet
Do not matches exactly with the IP address of one of its local interface
It means that the packet is not destined to that machine
but
Actually, the packet is destined to one of the machine present in the local subnet of this machine
So in this particular case, the machine forward the packet on to that particular subnet using L2 routing
so in the previous slide when the router B
Receives the packet from router A with destination IP address as 10.168.1.3
Now this destination IP address
do not matches with the IP address of any local interface of
the router B
So router B, do not consumes this packet
But the destination IP address is specified in the packet falls in the local subnet
of
10.168.1.0/24
So what will router B do? the router we will forward the packet
out of this interface and
Using l2 routing the packet eventually reaches its intended destination
Now you can see that on machine C
The IP address configured on the machine C
Exactly matches with the destination IP address is specified in the packet
Therefore, Machine C is the actual consumer of the packet
right!!
So these two points are important
Now the third point is that if none of the above conditions are true, in this case, machine forwards the packet using
l3 routing
to the next router
This is the first action that the router can take. the second action is the router can also choose to drop the packet
So when will router choose the action one or when will router choose to perform the action two?
We will study this in detail when we will be learning l3 routing
But for now just understand that if a router receives a packet such that the destination IP address of the packet
Do not matches exactly with the IP address of any of its local
interface
and
the packet neither falls in any
local subnet of the router, in this particular case the router can
Choose to forward the packet
Or it can choose to drop the packet
The router will choose to drop the packet if the destination IP address in the packet
belongs
to some remote subnet and
That particular remote sublet entry is present in l3 routing table of the router
Right and
if that particular entry is not present in L3 routing table of
The router then router do not know where to forward the packet
Right!!
so router is not the consumer of the packet and
the packet do not belong to any of the local subnet of the router and
third point is that the router do not contain the
Entry to the remote subnet to which the packet belongs
When all these three conditions are satisfied
then router actually do not know what to do with the packet and in this particular case the router choose to
drop the packet
So we will understand this concept in detail when we will be learning L3 routing
Now let us see the organization of a broadcast domain
So what is broadcast domain? A broadcast domain is nothing but a single wire
To which two or more than two machines are connected, right?
So such a wire is called broadcast domain
If there are only two machines connected that is machine A and machine B
Then this broadcast domain is more specifically called point to point domain or
point-to-point link
but if on a single wire
If more than two machines are connected then such a wire is referred to as broadcast domain
Right!!
So this is the broadcast domain in which the machines A, B, C and D are
present along with the router.
So, how does the subnet of devices look like?
So you can see that this pink colored region is actually a subnet
In which actually five devices are connected
Four devices are host machines that is A B C and D and
One of the interfaces of the router is also connected to this wire
So the second point says a subnet is always present behind the l3 router
now the point here to note here is that
Each interface of an L3 router actually represent a subnet
So you can see that this interface of an l3 router
Represent a subnet let's let us call this subnet as A
Right
Similarly, if there was another interface on the router then that particular interface would have represent another subnet
Let's say that subnet is B
so the point here to note is that each interface of an L3 router actually represent a subnet and
Depending on the IP address configured on that interface and the mask value configured on that interface
We can calculate the subnet ID
Right and all devices connected with the same wire
forms one subnet
So in our subnet A the machines or devices present are
Machine A machine B machine C machine D, and the router itself is also a member of this subnet
Now the next point is a
Packet sent by one machine is heard by all the machines in the same subnet
So, what does that mean? that suppose
One of the machine present in the same subnet
Places the packet on the wire and
Let us say that the destination of this packet is the Machine C which is present in the same subnet
So in this example, the communication is being carried out between the Machine A and machine C
now if the machine A places the packet on the wire
actually when the packet is placed on the wire, it means that machine A
places electrical signals
So electrical signals are nothing
Electrical signals are nothing but it is a varying voltage but it is a high and low
Voltage like this something like this
So high voltage represents a bit one and the low voltage represents a bit zero
so rest of the machines which are connected to the same wire, that is the Machine B and machine D actually
detects these varying voltage
or
Electrical signals which are being placed on the wire by the machine A
Right
So this is the property of the wire that if any two machines which are connected to the same wire communicate with each other
Then rest of the machines which are connected to the same wire also, hears the packet that are being transmitted
Because at the end of the day the packets are nothing but these are the electrical signals which are placed on the wire
now the machine B and machine D whether they want to detect those signals and
Wants to actually process those signals or not, It depends on certain criteria
So processing the electrical signals being detected on the wire is
Equivalent to processing the packet because these electrical signals are nothing but these are 1 0 1 0
bits which eventually represents a
Frame or packet that is being traversing on the wire
So depending on certain criteria of l2 routing machine B and machine D
May choose to ignore these electrical signals, or they may choose to process these electrical signals
So example is that all machines in your college campus may form one subnet
So this is the example of a subnet that all the machines of a hostel or all the machines in the same
department or all the machines in the same college campus may be present in the same subnet and
all those machines
Must be sitting behind the one main L3 router
So you can see in this diagram the machine A B c and D may represents a machine of your college campus
and
Those machines are has to be behind one l3 router which is called gateway router
So this is how the broadcast domain or
Multiple machines present in the same subnet are organized along with the l3 router
For example, the Machine B wants to communicate some machine
Let's say E
Which is present elsewhere in the internet
then all the data generated by the machine B will bypass the l3 router and
It has to go through the l3 router
through the internet. so
Whenever a machine in the one subnet wants to communicate with the machine
Present elsewhere that is out of the subnet then all the data transmission
Happens through the boundary or gateway l3 router of a subnet
So again, we will study all this formerly when we will be doing L2 routing an L3 routing in the
next module
so actually all this information is building up the foundation and
preparing us for
learning l2 routing and l3 routing in
subsequent modules
Also, the one point that I would like to discuss here is that
Whenever a machine which are present in different subnets wants to communicate with one another
Then the data has to be
Forwarded through l3 router
so in this example
let us say that there is some remote machine E present elsewhere in the network and let us
suppose the Machine C wants to communicate with the machine E
Now this is the example of communication where the communicating machines are present in different subnets
so all the data that is generated by the Machine C will have to go through the L3 router and
there could be any number of l3 routers present in the network and
eventually the data reaches the machine E
Right, so the point here I am trying to emphasize that whenever communicating machines communicate
whenever the communicating machines are present in different subnets, the data has to pass through the gateway l3 router of the subnet and
Whenever the communicating machines are present in the same subnet, then l3 router do not play any role
So L3 router is used only for L3 routing that is taking the data from one subnet to another subject
And
When the communicating machines belongs to the same subnet, then there is no role of l3 router being played in
Taking the traffic from source machine to destination machine which belongs to the same subnet.