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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 the 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 a remote
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device or a local device.
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So here's an example we have a PC with IP address 10 dot one that wanted that one and another PC with
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an IP address of 10 to one to two to one when the PC on the left wants to forward traffic to the PC
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on the right with the IP address tendered one to two that one.
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Does the PC for 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 are on different subnets the PCs will forward their traffic to their default gateways which
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will do the interview LAN routing.
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If they're on the local land or Local Area Network as an example all rocked the traffic if the traffic
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is forwarded to a traditional router.
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So at least three switch may do the routing between two villans way router my router the traffic between
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these two devices.
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If they are on different subnets but how would you determine if these two devices are on the same subnet
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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 10.0 wondered 1.1 and Tendo wonder 2.1 are in the same subnet if they are using a
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slash 16 mosque.
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However if they are using a 24 mosque that means that the devices are on different subnets.
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So let me explain that in more detail in Dittrich mosque allows a device to determine once again which
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portion of the address is the host portion 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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say PCa and PC be on the same subnet no default gateway is required.
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But if they are 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 mosques which are also known as natural mosques
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in a class A address the first octet is the network in a class B address the first two octets are network
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and with a Class C address the first three octets of the network portion.
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I'll explain more complicated subnet masks in the submitting videos.
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But let's first start with some simple examples.
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In this example we have a class a network that hasn't been submitted across a network the default mosque
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is to far 5.0 0.0.
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So if we look at a address such as 10 dot one to 1 one and convert that into binary it's going to look
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as follows.
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Now look at the following mosque 2 4 5 in binary equates to 8 binary ones 0 in decimal equates to 8
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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 1 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 tendered one but one that one is on network 10.00 at 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 10.0 1.1 one is on network 10 so when working out the network and host portions
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of an address follow these two simple rules any address bits that have a corresponding mosque that set
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to one in binary represents the network in the address bits that have a corresponding mosque but set
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to zero represents the host.
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So one in binary means network 0 in binary means host.
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So in this example 10 is the network because they are ones in the mosque in binary.
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So the network ID is set to 10.
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Notice these octets are populated by binary zeros that means host.
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So the host ID is equal to 1 dot 1.1.
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So in summary the network is 10.
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The host portion of the address is 1.1 1.
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Here's another example.
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Remember any address bits that have a corresponding mosque but set to 1 in binary represents makework
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any address bits that have a corresponding mosque but set to 0 in binary represent node ID.
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So here we've got a class A address.
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One got one that wanted one.
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But note the difference.
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The network mosque in this case is true 5 5 5.5 5.0 etc. so converting 1.1 at 1.1 to binary gives us
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the following.
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Taking the network mosque and converting that to binary gives us the following.
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Notice 205 equates to 8 binary ones which therefore means that those Portion of The address is a network
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so the network ID is one dot one and looking at the remaining part of the address which is populated
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with binary zeros in the network mosque means that one door one is the host portion of the address.
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In other words the network is one dot 1.0 to zero with a host portion of one.
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Got one on that network.
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The mosque is 2 5 5 5 0 0.
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In this example its easy to see the network portion of the address because we have 2 4 5 2 5 5 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 are seeing in these examples.
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You'll see that when we get to the submitting videos these two examples are simple because it's easy
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to recognize which portion is a network and which portion is host.
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In the submitting videos I'm going to show you much more complicated examples and in those examples
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it's more difficult to determine which portion is network and which portion is host.
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So in summary how does a device know whether another device is local 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 local
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to itself.
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If the network portion is not the same the local device knows that the other device is remote.
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