So, guys, going forward now, let us try to understand some of the drawbacks of local area networks.
The first drawback that we have because of LANs is immobility.
So what exactly that mean?
So you can see in this topology we have a L3 router here and one of the physical interface of this L3
router that is IFR host, a subnet whose network ID is 12 .1.1.0 slash 24.
Right.
So this is local area network and let us call this local area network as LAN one.
So you can see that in this LAN one there are three hosts already present.
Now, if I ask you one question, can you place the host D that is this machine in a different subnet
and host E and F in a different subnet, meaning that can you do something to place the host machine
D in a subnet which is different from LAN one?
Well, the simple rule of subnet says that all hosts have to be assigned an IP address which shares
the same network ID as that of Gateway router.
So in this case, you can see that all the hosts that is D, F and E are assigned an IP address such
that if you apply the mask value on this IP address, you will get the network ID.
Similarly, apply the mask value on an IP address configured on an interface of a gateway router.
You will obtain the same network ID.
So this is a necessary and compulsory conditions for the host to be present in the same subnet hosted
by a physical interface of a gateway router.
Now, if you want to move the machine to a different subnet, then obviously you would have to change
the IP address that is configured on the machine.
D.
Right.
But you cannot change the IP address of the machine De because it will violate this point.
For example, if you change the IP address of the machine D to 13 .1.1.2 slash 24, then it will going
to violate this point.
The network ID of this machine would be different from the network ID obtained by applying the mask
value on the IP address configured on a physical interface of a L3 router.
So this will be a wrong configuration.
So this is not the way to move the machine D out of subnet LAN one.
So the only way to move the host machine d to a different subnet is to physically move the host machine.
D So you can see that in the second diagram, the machine D has been moved physically from LAN one and
now it is connected to another physical interface of a L3 router and is now present in LAN two.
Now you are free to configure an IP address on the machine D, which is in compliance with the IP address
configured on the Gateway router interface.
Right.
So the question that I asked that can you place a host machine in a different subnet and host machine
E and F in a different subnet?
So to answer that question, you need to physically move the host machine D from LAN one to LAN two
meaning that you will have to re-enable the network in order to place the machines in different subnet.
And this is actually a big network management problem.
Just to move a machine from one subnet to a different subnet.
If you need to do recabling of the network, then it will be a big management problem for the network
administrators.
So you can see that to move the host machine d to a different subnet.
D needs to be physically moved behind the other interface of an L3 router.
Right.
So this is one problem of local area networks.
The second problem is that you are actually eating up the L3 routers interface just to place one host
in a different subnet.
Wasting L3 routers, real estate.
So you can see that in order to place the host machine D in a different subnet, you are actually consuming
one more physical interface of an L3 router.
And remember, L3 routers are very expensive machines and they come with very limited physical interfaces.
If you consume physical interface of an L3 router just for the purpose of creating subnets, then it
will be actually under-utilization of L3 routers.
In other words, it will be a waste usage of l3 router as a device.
L3 routers are actually very expensive devices and they come with limited number of physical interfaces.
So you understand the problem here, that local area networks do not allow the flexibility in the sense
that host machines can be moved from one subnet to another subnet without actually requiring to re-enable
the network.
So let us try to understand the real world implications of this problem.