So guys going forward let us try to understand
the destination IP address in real sense
Let us suppose the l3 router A wants to send data with destination IP address X
where X could be
Then B will consume the data
That is if router A sends the data with destination IP address
Is equal to IP address of any local interface of router B
Then, when router B will receives the data it will going to consume the data
Consume the data means the layer 3 of the tcp/ip stack
Will conclude that the data is destined for router B
and it will hand over the rest of the packet to the transport layer or
Application layer running above the network layer in the tcp/ip stack
So you can see that the router A can send the data to the router B by
Specifying the destination IP address as IP address of any local interface of router B
So this is the criteria of
receiving the data and consuming the data by a machine that is
Destination IP address must be equal to the IP address of any local interface of a receiving machine
But what if A sends the data with destination IP address X
Where X is equal to for example 10.168.1.3
Right
so
In this scenario, the l3 router B will going to receive the data, but it will not consume the data
rather the router
B will use l2 routing to further send the data to the Machine C
Right!! and when the Machine C receives the data
It will check that the destination IP address in the IP header is equal to the IP address of its local interface
So this condition becomes true and the Machine C will going to consume the data
Right, so router B when receives the data
such that the destination IP address is
Not equal to any IP address of local interface of the router B
but the destination IP address
falls in one of the local subnet of l3 router
B
Then router B will
Forward the data further in its local subnet to which the data is
destined
these two are basic principles that clarifies or which states the criteria of
receiving and consuming the data or
further
propagating the data
To the Machine present in the local subnet attached to the l3 router
So if condition one is satisfied the data consume
The data will be consumed and if this condition is specified
then l3 router further
Forwards the data using l2 routing in
its local subnet
So l3 routers talk with each other via l3 routing only
So L3 routers talk to each other using l3 routing only
Whereas l3 routers talk with machines present in its local subnet
using l2 routing
so in our example if the router A sends the data with destination IP address
Equal to 10.168.1.3
Then the data will reach
gateway router B using l3 routing
because the destination machine
Which is supposed to receive the data. That is the Machine C is
present in one of the local subnet of router B
So you can see that the journey of the packet is divided into
two phases here
The first phase is the L3 routing using L3 routing the packet eventually reach
the destination gateway router and
In the second phase the destination gateway router
Uses l2 routing to eventually deliver the packet to the destination
machine
Right
also note the point here that
each interface of l3 router is a
subnet
So you can see that in this example
This interface represents a subnet
192.168.1.0/24 and
this interface represent a subnet 1
193.168.1.0/24 and
So on