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So in this example of these devices in the same subnet or in different subnets the PC on the left has
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an IP address of 10 dot one at one but one with the subnet mask of 2 4 5 2 4 5 0 0 the device on the
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right has an IP address of 10 dot wander to that one with a Mosque of 5.2 for 5.0 to zero.
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So the device on the left does the following when it wants to communicate with the device on the right.
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It does a logical and on the network portion of the address so first determines which portion of its
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address is the network portion and then it compares that to the network portion of the device that it's
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wanting to communicate with.
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So in this example the network portion is ten to one.
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So in other words the first two octets of the address on network the device checks the first two octets
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of the other device to see if it's the same as its local network portion.
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And in this example they all the same.
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So the device on the left will send traffic to the device on the right directly and it will not try
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and send the traffic to its default gateway.
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What it will do is send an up message onto the local segment requesting the MAC address associated with
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IP address 10 1 to two to 1.
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It will try and communicate with 10.0 one or two to one on the local segment directly and not send the
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traffic to a default gateway.
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And the reason for that is that the network portion of the addresses are the same.
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So the local device knows that the other device is on the same or local segment as itself.
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In this example the network mosque has changed its 2.5 to 4 5 2 5 5 0.
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So the device on the left will do a logical and and check whether the network portion of the device
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that it's wanting to communicate with is the same as its network portion.
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So based on the subnet mask the local devices network portion is 10 dot 1.1.
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The device it's wanting to communicate with has a network portion of 10 1 or two.
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So in this case the network portion is different.
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So the local host knows that the device that it's wanting to communicate with is on a different subnet
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to itself.
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So because these two devices on different subnets the local device will send its traffic to its configured
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default gateway PCs will send traffic to their default gateways when a default gateway is configured.
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In this example let's assume that a default gateway is configured and that's normally what happens in
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a real world implementation.
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So the PC is trying to talk to a device in a different subnet so it'll same it's traffic to its default
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gateway.
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So in summary the subnet mask allows the local device to determine whether the device that it's trying
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to communicate with is on the same subnet as itself or if it's on a different subnet.
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Now Cisco and most network vendors do not support discontiguous subnet mosques a discontiguous subnet
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mask would look something like the following.
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Notice in the binary we have binary ones then binary zeros then binary ones binary zeros binary ones
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binary zeros and so forth and so on.
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This type of discontiguous subnet mask is not supported only contiguous subnet masks are supported in
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this example.
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We have contiguous or continuous ones in the binary and then contiguous or continuous zeros in the binary.
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Converting that to decimal gives us a value of 255 242 at 0.0.
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In this example notice we have contiguous ones in binary and then contiguous zeros in binary giving
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us a result in decimal of 2 5 5 2 5 5 8 1 9 2 0.
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So in a subnet mask we must start with binary ones and they must be contiguous.
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So you cannot have binary ones then binary zeros then binary ones and so forth and so one they must
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be contiguous ones and then contiguous zeros.
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So you can for argument's sake have a subnet mask something like 0.00 240.
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That's not supported subnet masks have to be contiguous ones followed by contiguous zeros.
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Discontiguous subnet masks are not supported and think goodness for that because it makes our lives
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as a network engineer is a lot easier to have contiguous subnet masks.
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