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Now, what about subnet masks, 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
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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, that one, that one and another PC with an IP address of 10.1
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to Tutut one when the PC on the left wants to forward traffic to the PC on the right with IP address.
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10.1 the two that one does the PC for the traffic on to the local segment or does it forward it to its
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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,
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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 the traffic if the traffic is forwarded to a traditional router.
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So a layer three switch may do.
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The routing between two villans way router may route the traffic between these two devices if they are
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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?
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I'm going to explain this in more detail in the next few minutes.
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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
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a slash 16 mosque.
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However, if they're 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.
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A network mosque 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, or if the device is on the local
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subnet and therefore does not require the use of a default gateway.
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So S.P.C.A. and PTB are in the same subnet.
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No default gateway is required, but if they are on different subnets, then a default gateway would
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typically be required to do the routing between the two pieces.
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So that's essentially what a network mosque 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
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mosques.
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In a class I address, the first octet is the network in a class B address, the first talking octets
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or network, and with a classy address, the first three octets of the network portion.
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I'll explain more complicated subnet masks in the subnet 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 in a class A network.
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The default mosque is too far, 5.0 or 020.
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So if we look at a address such as 10.1, that one, that one, and convert that into binary, it's
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going to look as follows.
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Now, look at the following mosque, 2005 in binary equates to eight binary ones, zero in decimal equates
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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 network and this portion of the address is host.
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Hence this device with IP address.
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10.1 that one.
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That one is on network ten dot 0.01 zero.
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This is the network portion and this is the host portion.
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This device with IP address 10.1 not one dot one is on Network ten.
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So when working out the network and hosting 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.
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Notice 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 one one one.
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So in summary, the network is 10.
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The host portion of the address is one dot one one.
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Here's another example.
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Remember any address bits that have a corresponding mosque but six 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 address.
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One dot one.
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That one.
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That one.
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But note the difference.
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The network mosque in this case is to find 5.2 5.0.
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That's zero.
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So converting one dot one, that one good one to binary gives us the following.
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Taking the network mosque and converting that to binary gives us the following notice.
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To find five equates to eight binary ones, which therefore means that this portion of the address is
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a network.
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So the network ID is one dot one.
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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 one dot one is the host portion of the address.
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In other words, the network is 1.0 1.0 arrow with a host portion of one dot one on that network.
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The mosque is too 5.5 5.0 to zero.
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In this example, it's easy to see the network portion of the address because we have to 5.5 five in
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the 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, these two examples are simple because it's easy to
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recognize which portion is a network and which portion is host in the subediting 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
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is 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, if the network portion of the address is the same, the local device
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knows that the other device is 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.
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