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Now, what about subnet masks?

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I've mentioned subnet masks a few times.

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What is a subnet mask?

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What does it do?

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

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part of the address is the host portion.

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

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a remote device or a local device.

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So here's an example.

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We have a PC with IP address, 10.1, 1.1 and another PC with an IP address of 10.1 to 2.1.

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When the PC on the left wants to forward traffic to the PC on the right with IP address 10.1 to 2.1,

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does the PC forward the traffic onto the local segment or does it forward it to its default gateway?

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

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gateway.

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

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

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

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So a layer three switch may do the routing between two VLANs or a router may route the traffic between

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these two devices if they are in different subnets.

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

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

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But as an example, 10.1, .1.1 and 10.1 or 2.1 are in the same subnet if they're using a slash 16 mask.

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

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So let me explain that in more detail.

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

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and which portion of the address is the network portion.

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

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on a remote network and is thus reachable via the default gateway.

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Or if the device is on the local subnet and therefore does not require the use of a default gateway.

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So if PCA and PCB are in the same subnet, no default gateway is required.

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But if they're on different subnets, then a default gateway would typically be required to do the routing

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between the two PCs.

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So that's essentially what a network mask does.

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

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masks in a Class A address.

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The first octet is the network in a Class B address.

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The first two octets are network and with a class C address, the first three octets are the network

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portion.

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I'll explain more complicated subnet masks in the subnet videos, but let's first start with some simple

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examples.

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

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The default mask is 255.0.0.0.

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So if we look at a address such as 10.1, 1.1 and convert that into binary, it's going to look as follows.

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Now look at the following mosque 255 in binary equates to eight binary ones.

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Zero in decimal equates to eight binary zeros.

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

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continuous ones, starting from the left hand side.

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

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host.

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

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Hence this device with IP address 10.1 1.1 is on network ten .0.0.0.

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

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This device with IP address ten .1.1.1 is on network ten.

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

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

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

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

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

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So the network ID is set to ten notice.

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These octets are populated by binary zeros.

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That means host.

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So the host ID is equal to 1.1.1.

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So in summary, the network is ten.

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The host portion of the address is 1.1.1.

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He has another example.

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

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

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So here we've got a class A address 1.1.1.1.

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But note the difference.

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The network mosque in this case is 255225500.

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So converting 1.1.1.1 to binary gives us the following.

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Taking the network mosque and converting that to binary gives us the following notice 255 equates to

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eight binary ones, which therefore means that this portion of the address is a network.

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So the network ID is 1.1.

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

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mosque, means that 1.1 is the host portion of the address.

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In other words, the network is 1.1.0.0 with a host portion of 1.1 on that network.

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The mosque is 255.255.020.

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In this example, it's easy to see the network portion of the address because we have 255.255 in the

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network mosque.

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

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You'll see that when we get to the subnet videos.

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These two examples are simple because it's easy to recognize which portion is a network and which portion

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is host in the subnet videos.

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

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

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host.

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

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

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to the address of the other host.

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If the network portion of the address is the same, the local device knows that the other device is

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local to itself.

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