So, welcome back guys. In this lecture video, we will try to understand the valid and invalid
Configuration scenarios for assigning ip addresses
So in this topology you can see that you have a l3 router, which have two subnets on its two interfaces
the subnet on the left hand side is
10.1.1.0/24
and a subnet on the right hand side 11.1.1.0/24
So having understood the ip address mathematics, it must be pretty much clear to you how to calculate the
Network, id or subnet address?
Now you can see that the subnet on the right hand side that is this subnet
Consists of 4 machines that is machine A machine B machine C And machine D
Right and you can, also confirm that each of these machines are present in the same subnet
And that subnet is 11.1.1.0/24
right
So everything looks perfect
Now, what if you change the ip address of the machine C to
11.1.2.4/24
what will happen? is it allowed? so let's see
so if you change the ip address of the machine C
To ip address 11.1.
2.4/
24 then c's network, id will be
11.1.2.0/24
whereas rest of the machines network, id will be 11.1.1.0/24
so if you change the ip address of the machine C
To 11.1. 2.
4/24 then C will move from the subnet
11.1.1.0/24 to a new subnet 11.
1.2.0/
24
So it means if you change the ip address of the machine C, then the machine A machine B and machine D
Will be present in one subnet whereas the machine C will be present in a different subnet
Though all of the machines are physically connected to the same wire
so if you change the ip address of the machine C as
We discussed, C will now be present in a different subnet all together
so two subnets must always be
Separated note the word separated by L3 router
So remember L3 router is required to route the traffic from one subnet to another
Since you have changed the ip address configuration of the machine C, Machine C is
Present in one subnet whereas machine A machine B and machine D are present in different subnet
right now the subnet of the machine C is
Not separated by the subnet of the machine A B and D by entry router in between
right so
Emphasizing on this point once again
two subnet must be separated by a L3 router so if you have a subnet A and you have a subnet B
Then you must have an L3 router in between which connects the two subnet
Now in this example
Machine C and the machine A B D are not separated by L3 router
So in this example there is no L3 router present between C and the rest of the machines
So what will happen, C's data cannot be delivered to any of the machine A B or D or the router itself
Because to route the traffic out of C's subnet L3 router is required
right
so you can, visualize this scenario as two islands that, is sub networks not connected through sea route
So hence this configuration is illegal
So conclusion is that each router's interface is connected to one subnet or another L3 router
Hence each router interface ip mask value must yield unique network
Id which identifies the subnet on that interface
any two machines
Whose ip and mask value
yields the same network id are said to be present in the same subnet
right so
Because you have changed the ip address of the machine C, the machine c has now moved from subnet 11.1.1.
0/24 to a new subnet which is 11.1
2.0/24. now in order to carry out the communication between C and the rest of the machine
After changing the ip address of the machine C, you have to introduce a new L3 router between the machine C and the rest of the machine
Right if you do this then machine then only machine C will be able to communicate with rest of the machines
So you can redesign this topology like
This you have an L3 router the same L3 router then you have a wired connection you have the machine A
you have the machine B here and
Then you have a L3 router here and to this L3 router
machine C is connected and you have machine D
Right and the machine C's ip address s 11.1.2.4/24 and
the ip address on the other end of the router will be
11.1.1.6/24
right so you can see that machine A machine B and this interface of
new router and the machine D
Now all falls in the same subnet, which is 11.1.1.
0/24
Whereas the machine C is now been separated from the
Subnet of machine A B and D via new l3 router
So if machine C
Wants to send a data to machine A, C now depends on l3 routing by L3 router
right
So i hope you understand if you change the ip address of any machines such that
That machine do not belongs to that particular subnet anymore then that machine has to be separated by a new L3 router
in order to facilitate the communication between
The machine C and the rest of the machines
So in this example because you have changed the ip address of the machine C
Which causes the machine C to move to a new subnet you have to introduce a new L3 Router to compensate for that
So now going forward let us discuss the another example of invalid configuration of ip addresses
Suppose you have a L3 router and
You can see that in this example the l3 router has three interfaces now
If you analyze the ip addresses
Which are configured on the interfaces of this router you see that the interface 2 and the interface 3
Both are present in the same subnet
Which has the network id 11.1.1.
0/24
Right so going, by the principle that
Different subnets must be separated by a l3 router and same subnet should not be separated
By a L3 router so in this example you have a subnet 11.1.1
0/24
On this interface and you have the same subnet, which is 11.1.1.
0/24 on this interface so
In this example the same subnet is now separatedby a L3router
Right because two different
interfaces of the same
Router have the same network id which is
Illegal, this is invalid configuration
so conclusion
Is that we cannot assign ip address or mask value
Which evaluates to same network id to multiple interfaces of a l3 router
You should view l3 router as junction of
Different subnets. in this example, we are trying to use l3 router as junction of same subnet
Because, we have configured different subnets on two different interfaces of a L3 Router, which is illegal configuration
Every interface of a l3 Router must evaluate to a unique network id
So this is another example of mis configuration of ip addresses