so guys next we will try to understand the recommended mask value to be used
for p2p links. So it is a very common scenario in networking that two devices
say L3 routers are connected using point-to-point links. So it is actually a
very common scenario or a very common deployment that the two machines which
could be L3 routers are connected by a point-to-point link. So point-to-point
links are those links which have only Two end interfaces and there is no
scope or there is no room to connect the third device to this link. Right ? So that
is why they are called point-to-point link because they have only two points
to which the devices can be connected. So now you can see that in this deployment
there are two routers, Router1 and Router2. Right ? so the subnet ID or the
subnet of this point-to-point link is 11.1.1.0/24. right ? Now note
that the mask value is 24. So going forward the two interfaces of the two
routers have the same network ID, and what is that network ID ? The network ID
is 11.1.1.0/24. Therefore the two interfaces of the
two routers are in the same subnet. why they are in the same subnet ? Because if
you apply the mask value on the IP address on both the interfaces of the
two routers you will get the same network ID. So this is the criteria to
decide whether the two interfaces are in the same subnet or different subnet. So
we have already discussed regarding this. so point-to-point ends are always in the
same subnet. So if you configure an IP address here such that it evaluates the
network ID say ID 1 and if you configure an IP address on
this side of the interface such that the network ID evaluated is ID 2 - and if ID 1
is not equal to ID 2 - then this configuration will be invalid. The IP
address that are assigned to the two ends of a point-to-point links should
always evaluate to same network ID. Right ? so point-to-point link is actually a
straight wire which connects the two ends. So there is no room to connect the
third device to the subnet. Right ? So this looks very obvious. Now in our example
you can see that the mask value being used is 24. So now you already know how
to calculate the maximum number of machines that could be present in this
subnet. The mask value 24 allows you to connect 254 machines to the subnet. Right ?
so theoretically you can connect 254 machines to this subnet. But practically
since it's a point-to-point link which allows you to connect only two devices.
so practically you can connect only two devices but theoretically the subnet or
the mask value allows you to connect 254 devices. So I use a /24 as a mask. we
should use the mask value which strictly allows only two devices to be present in
the same subnet in case the link being used is a point-to-point link so
such a mask value is 30 thus in case of point-to-point links
while assigning an IP address to the ends of the link mask value 30 is
usually used. though using a mask value say / 24 is also a valid configuration
for a point link, but it is usually a
recommendation to use a mask value of 30 while assigning an IP addresses to the
end of a point-to-point links. so you can use the mask value / 30 on both the
sides of the point-to-point link while assigning an IP addresses. Right ? So it is
just a good practice to use / 30 as a mask value for point-to-point subnets
now let us try to understand the point that why the mask value of 30 strictly
allows only two devices to be present in the subnet. So you can see that if you
use the mask value 30 you have only two control bits and using two control bits
you can form only four different permutations of two bits which is 0 0 0
1 1 0 and 1 1 the 0 0 corresponds to network ID and 11 corresponds to
broadcast ID. Right ? So there are only two combination of control bits which are
left to be used by assignable IP addresses. So you can see that if the
mask value is 30 then only two IP addresses could be possible to be
assigned to two different devices on a point-to-point link. The two possible IP
addresses are 11.1.1.1 and 11.1.1.2 . Right ? so
hence only two interfaces can be present in the subnet and you have one network
ID and one broadcast address. note that the broadcast address now is
11.1.1.3/30 so why you are getting three here because the two
control bits are 1 and 1, here, and 11 corresponds to 3.
Right ? So the broadcast address will become 11.1.1.3/30. So I hope I am
able to make the point clear that using the mask value 30 will strictly allow
you to use only two IP addresses to be assigned to the two ends of the link and
you will have one network ID and one broadcast ID or broadcast address as
usual. Right ?