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The role of a switch
is to forward traffic

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based on the
destination MAC address

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inside of an ethernet frame.

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This means the switch needs to
keep an ongoing and active list

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of all of the devices
it happens to know about

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based on the MAC address
of those devices.

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The switch builds this list
by looking at inbound traffic

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and examining the
source MAC address,

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and tying that source MAC address

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to a specific physical interface.

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And for switches
that are configured

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with spanning tree
protocol, or STP,

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they're also responsible for
ensuring that a loop does not

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occur on the switch network.

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The process of sending traffic
through a switch network

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is the same for every ethernet frame.

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Let's take this
scenario where Sam,

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and you can see the MAC
address for Sam's device,

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is sending information
to the SGC server,

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and you can see the SGC server's
MAC address is 1000:5555:5555.

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We have a switch in the
middle, and all of our devices

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are plugged into the switch,
including Sam and the SGC server.

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Inside of the switch
is a MAC address table.

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It lists all the MAC
addresses and the interfaces

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where those addresses are connected.

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When Sam sends traffic to the
switch with the destination MAC

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address of
1000:5555:5555, the switch

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looks up that address in its table,

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and if one matches one of the
entries inside of that table,

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it identifies the output
interface for that traffic

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and sends it down that
interface to the server

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that has that MAC address.

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If you have multiple switches,
it's exactly the same process,

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except it occurs twice,
once on the first switch

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and once on the second.

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You can see this is the
same configuration where

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Sam is communicating
to the SGC server,

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but there is a switch A
on one side of the network,

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and a switch B on the other.

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Switch A has a MAC address table
specific to the devices plugged

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into Switch A, and Switch B
has a completely different

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and unique, MAC address table.

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Sam is going to send traffic
again to the SGC server.

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It knows that it's sending this traffic 
to MAC address 1000:5555:5555.

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As that traffic hits Switch A, Switch A

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refers to its own MAC address table

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and knows that that particular
MAC address is located

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on an interface that is
a gigabit 0/2 interface,

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and so it sends that
traffic out that interface

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to the next switch.

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On that switch, the
same lookup process

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occurs, where Switch B will
examine the destination

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MAC address, determine that
that MAC address is associated

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with the interface fast ethernet 0/5,

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and sends that traffic down that
interface to the destination device.

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You can see that building 
that MAC address table

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is extremely important.

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If we didn't have the MAC 
address table, the switch

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would not know where to send that traffic.

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In order to build
that table, the switch

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is going to examine
all incoming traffic

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and make a note of the
source MAC address.

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It will then associate
that source MAC address

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to a specific interface
on the switch.

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So let's take a scenario where
we've just powered up a switch,

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it has nothing in the
MAC address table,

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and we're going to send
information from Sam's computer

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to the SGC server.

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Sam's going to send that
traffic to the switch,

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the switch is going to examine
the source MAC address,

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and in the case of Sam's
device, that's 1000:1111:1111.

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It will then put that MAC
address into the MAC address

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table, and it will
identify the interface

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where that information was received.
In this case, interface F0/1.

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That information is then
sent on to the SGC server,

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and then when the SGC server
responds to that communication,

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it has a different
source MAC address,

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and the process is repeated.
Except in this case,

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the switch identifies
that MAC address is

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coming from fast ethernet 0/5.

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In that previous example, we 
were sending information to the SGC

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server, but the SGC 
server's MAC address

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was not yet in the switch.

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If the switch does not have
an entry for that MAC address

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in the table, then it
will send that information

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to everyone on the network.

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For example, we'll take Sam
sending this information

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to the SGC server.

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You can see in this case,
the MAC address table

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has nothing inside
of it at the moment.

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The MAC address table will be
updated with the source MAC

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address because Sam did send
that information to the switch,

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and it did associate that
with fast ethernet 0/1,

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But we're sending this information

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to a destination MAC
address that's not currently

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listed in the switch's table.

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In that case, it's going to now
send that traffic to everybody

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on the network, and
effectively flood

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that traffic to all of the
other interfaces on that switch.

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If you're familiar with
the operation of a hub,

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then you'll notice that this
is very similar to the way

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a hub works normally.

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But this traffic being
sent to every device

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ensures that at least
the destination will

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receive this particular frame.

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And in this example, you can see
that the SGC server did indeed

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receive that frame, and
when the SGC server responds

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back to Sam with a response,
the source MAC address

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will be identified by the switch.

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That information will be added
to the MAC address table,

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and the switch will no longer 
need to flood the traffic

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across all interfaces if communication
is occurring between Sam

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and the SGC server again.

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On an IPv4 network,
devices are able to obtain

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the MAC address of a remote
device using the ARP protocol.

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ARP stands for 
Address Resolution Protocol.

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ARP will query the network
for a specific IP address,

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and that IP address will respond
back with its MAC address.

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Your local computer keeps
a cache of all of the MAC

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addresses that it currently knows.

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If you wanted to look at 
the ARP address table on

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your local machine, you 
can use the command arp-a.

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Let's run the arp-a
command on my machine.

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You can see that I have a number
of local devices on the 10.1.10 network.

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You can see them all listed here.

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There's also some other
devices on my local network,

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including some APIPA addresses
and some multicast addresses.

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Let's say that I want to
communicate to a switch

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that I have on my network.

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That switch's IP
address is 10.1.10.210,

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and you can see in
my ARP address table,

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I don't currently have
that address in the list.

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So I'm going to perform a ping,
and I'm gonna ping 10.1.10.210,

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and I'll get some responses back
from that particular device.

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If I now look at my ARP
address table with an arp-a,

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you will see that I have a
new entry for 10.1.10.210,

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and you'll see that I have a MAC
address associated with that IP address.

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When I performed that ping,
the first thing that occurred

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was an ARP request
made to the network

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to try to find that
particular device,

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and I received an
ARP response, which

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then allowed me to send traffic
to that device directly.

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I captured the ARP
communication using Wireshark,

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which is a packet analyzer, and
you can download and install

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Wireshark on your own
machine to see not only ARPs,

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but all of the network
traffic on your system.

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The first frame that I'm
sending is from my device,

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and it's sending it
out as a broadcast,

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and the ARP itself is
requesting the MAC address

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for who has 10.1.10.210.

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You can see the
details of the ARP

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that are located further
down in the detail.

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You can see the
sender MAC address,

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which is my Apple computer.

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You can see my local IP
address, which is 10.1.10.249.

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You can see the
target MAC address,

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right now we don't know what
the MAC address is of the target,

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so it's all zeros, and you can
see that I'm requesting the MAC

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address for the device that has
the IP address of 10.1.10.210.

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We very quickly get a response
from this device, which

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happens to be a Cisco
switch, and the response

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from the MAC address is
from the Cisco MAC address

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with the sender's IP address,
which is 10.1.10.210,

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and the target is the response back

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to my Apple computer
and my local IP address.

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You can see in the response
that it filled in the sender MAC address,

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so instead of being all zeros,
I see this long MAC address

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associated with this IP.

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And if you remember
the IP address and MAC

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address in my
local ARP cache, it

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matches both of those that were
received by this ARP response.

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That ARP process is what
we use an IP version

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4 to be able to
identify a MAC address,

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but we don't have
broadcasts in IPv6.

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There's also a different
process for IPv6

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to identify the MAC addresses of
devices on your local network.

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In IPv6, we use in NDP,
which is Neighbor Discovery

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Protocol, using multicast,
specifically with ICMPv6.

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This replaces the ARP function
that we would commonly

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see in IPv4 with
this Neighbor MAC Discovery.

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This can also be
used in conjunction

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with SLAAC, which is Stateless
Address Autoconfiguration,

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which allows the system to
automatically configure itself

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with an IP address without
using a DHCP server.

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Neighbor Discovery Protocol is also

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used to identify any
duplicate addresses using DAD,

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or Duplicate Address Detection.

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If you wanted to see
the conversation that

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takes place in IPv6,
instead of using ARP,

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we send a neighbor solicitation,
or NS, on a multicast address,

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and that is the IPv6 multicast that's 
used for this neighbor solicitation frame.

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The response is sent back from
the other side with a neighbor

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advertisement, or NA, and that NA

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includes the MAC address
of that local device.

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Although the protocols and the
method is slightly different,

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you can see that the process is very 
similar to the one that occurs in IPv4.

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Not only are we sending
data over ethernet networks,

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we can also send power
over those networks

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at the same time using
Power over Ethernet, or POE.

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00:10:11,860 --> 00:10:15,610
This allows us to connect
devices such as access points,

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voiceover IP phones,
and other devices

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by simply plugging in
an ethernet connection.

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00:10:21,370 --> 00:10:23,920
You don't have to then
plug in a separate power

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connection for that device.

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That power is coming
from either the switch,

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or another device that's
connected into the network.

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If it's coming from the switch, 
we call that an Endspan,

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or if it's coming from an injector,
like the one you see here,

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which sits in the middle of an
existing ethernet connection,

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00:10:41,020 --> 00:10:44,080
we refer to that as a Midspan.

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00:10:44,080 --> 00:10:47,260
If your ethernet network
is a 10 or 100 megabit

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00:10:47,260 --> 00:10:51,010
per second connection, then you
have some extra wires inside

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00:10:51,010 --> 00:10:53,110
of that cable that you
could use for power.

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00:10:53,110 --> 00:10:56,290
We refer to that as Mode
B power over ethernet,

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00:10:56,290 --> 00:10:58,990
where you're sending
power on the spare pairs.

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But if you're using
gigabit connections,

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00:11:01,030 --> 00:11:04,660
you're using all of those wires
for your gigabit ethernet data.

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00:11:04,660 --> 00:11:06,970
And in those cases,
we're using Mode A,

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00:11:06,970 --> 00:11:11,108
where we're sending power
and data over the same wire.

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00:11:11,108 --> 00:11:13,400
You'll find there are a
number of different power

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00:11:13,400 --> 00:11:15,730
over ethernet standards,
and these standards

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are being added to and
changed all the time.

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00:11:18,460 --> 00:11:24,370
Two very common standards are
the IEEE 802.3af from 2003.

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00:11:24,370 --> 00:11:28,000
We refer to that as the
original POE standard, which

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00:11:28,000 --> 00:11:32,320
provides 15.4 watts of
direct current power,

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00:11:32,320 --> 00:11:35,230
with a maximum current
of 350 milliamps.

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00:11:35,230 --> 00:11:38,290
An update to that standard
is what we call POE+.

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This was updated
with 802.3at in 2009.

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00:11:42,640 --> 00:11:48,040
This also has been incorporated
into the existing 802.3 ethernet standard.

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This provides a bit more
power on the ethernet network,

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00:11:50,920 --> 00:11:57,100
25.5 watts of DC power, with a 
maximum current of 600 milliamps.

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There are other power
over ethernet standards,

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00:11:59,490 --> 00:12:01,430
and these are being
updated all the time,

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00:12:01,430 --> 00:12:04,990
so make sure you check with the
ethernet standards from IEEE

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00:12:04,990 --> 00:12:09,570
to know exactly what options
may be available for you.