Original subtitles

Now, what about subnet masks, I've mentioned subnet masks a few times.

What is the subnet mask?

What does it do?

Well, a subnet mask is used to determine which part of an IP address is the network portion and which

part of the address is the host portion.

This allows a PC, for example, to determine whether a device that it wants to communicate with is

a remote device or a local device.

So here's an example.

We have a PC with IP address, 10.1, that one, that one and another PC with an IP address of 10.1

to Tutut one when the PC on the left wants to forward traffic to the PC on the right with IP address.

10.1 the two that one does the PC for the traffic on to the local segment or does it forward it to its

default gateway?

If these two devices are in the same subnet, they can communicate directly without the use of a default

gateway.

But if they are on different subnets, the PCs will forward their traffic to their default gateways,

which will do the inter VLAN routing if they're on the local LAN or local area network as an example,

or the traffic if the traffic is forwarded to a traditional router.

So a layer three switch may do.

The routing between two villans way router may route the traffic between these two devices if they are

in different subnets.

But how would you determine if these two devices are on the same subnet or if they're on different subnets

now?

I'm going to explain this in more detail in the next few minutes.

But as an example, 10.1 dot dot one and 10.1 or two dot one are in the same subnet if they are using

a slash 16 mosque.

However, if they're using a 24 mosque, that means that the devices are on different subnets.

So let me explain that in more detail.

A network mosque allows a device to determine once again which portion of the address is the host portion

and which portion of the address is the network portion.

This allows a local PC as an example to determine whether the device it wants to communicate with is

on a remote network and is thus reachable via the default gateway, or if the device is on the local

subnet and therefore does not require the use of a default gateway.

So S.P.C.A. and PTB are in the same subnet.

No default gateway is required, but if they are on different subnets, then a default gateway would

typically be required to do the routing between the two pieces.

So that's essentially what a network mosque does.

Now, as I've explained, plus A, B and C networks have default mosques, which are also known as natural

mosques.

In a class I address, the first octet is the network in a class B address, the first talking octets

or network, and with a classy address, the first three octets of the network portion.

I'll explain more complicated subnet masks in the subnet videos.

But let's first start with some simple examples.

In this example, we have a class A network that hasn't been submitted in a class A network.

The default mosque is too far, 5.0 or 020.

So if we look at a address such as 10.1, that one, that one, and convert that into binary, it's

going to look as follows.

Now, look at the following mosque, 2005 in binary equates to eight binary ones, zero in decimal equates

to eight binary zeros.

So converting the mosque into binary shows us that the network portion consists of contiguous ones or

continuous ones, starting from the left hand side.

A one in binary in the network mosque indicates network a zero in binary in the network mosque indicates

host.

So in this example, this portion of the address is network and this portion of the address is host.

Hence this device with IP address.

10.1 that one.

That one is on network ten dot 0.01 zero.

This is the network portion and this is the host portion.

This device with IP address 10.1 not one dot one is on Network ten.

So when working out the network and hosting portions of an address, follow these two simple rules.

Any address bits that have a corresponding mosque but set to one in binary represents the network.

Any address bits that have a corresponding mosque but set to zero represents the host.

So one in binary means network zero in binary means host.

So in this example, ten is the network because there are ones in the mosque in binary.

So the network ID is set to ten.

Notice these octets are populated by binary zeros.

That means host.

So the host ID is equal to one one one.

So in summary, the network is 10.

The host portion of the address is one dot one one.

Here's another example.

Remember any address bits that have a corresponding mosque but six to one in binary represents network.

Any address, bits that have a corresponding mosque but set to zero in binary represent node ID.

So here we've got a class address.

One dot one.

That one.

That one.

But note the difference.

The network mosque in this case is to find 5.2 5.0.

That's zero.

So converting one dot one, that one good one to binary gives us the following.

Taking the network mosque and converting that to binary gives us the following notice.

To find five equates to eight binary ones, which therefore means that this portion of the address is

a network.

So the network ID is one dot one.

And looking at the remaining part of the address, which is populated with binary zeros in the network

mosque, means that one dot one is the host portion of the address.

In other words, the network is 1.0 1.0 arrow with a host portion of one dot one on that network.

The mosque is too 5.5 5.0 to zero.

In this example, it's easy to see the network portion of the address because we have to 5.5 five in

the network mosque.

Just be aware that things can get a lot more complicated than what we're seeing in these examples.

You'll see that when we get to the subnet videos, these two examples are simple because it's easy to

recognize which portion is a network and which portion is host in the subediting videos.

I'm going to show you much more complicated examples.

And in those examples, it's more difficult to determine which portion is network and which portion

is host.

So in summary, how does a device know whether another device is local or remote to itself?

So the first thing it will do is check the network portion of its local address and then compare that

to the address of the other host, if the network portion of the address is the same, the local device

knows that the other device is local to itself.

If the network portion is not the same, the local device knows that the other device is remote.