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In the previous video, we have done configuring BGP on google and ISP.

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Next in this video, we are going to learn about the concept of
network address translation.

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But before that, let’s remember about private
and public ip address first.

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Public ip address is in the internet and accessible
from anywhere, but private ip address is only

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accessible from the local network.

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In this topology, let’s say the Ip address
of google server, the network between google

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and ISP, and the network between ISP and customer
is public ip address.

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And then the ip address of vlan 10 and vlan
20 is private ip address.

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The problem is, private ip address will not
able to go to the internet, so for example

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here from vlan 10 want to ping to google,
so the source address is 192.168.10.2,

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let's say this is the ip address of pc 0, and then
the destination will be the google,

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which is 172.16.10.2.

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When pc 0 send this packet, the packet will
go through router 3, and then forwarded to ISP.

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ISP will see the packet, here ISP will see
that the source address of the packet is 192.168.10.2,

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ISP don’t know about this network, because
this is private ip address,

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so the packet will not forwarded to the internet or google.

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So for now, the client will not able to communicate
with internet or google.

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So the solution is, we will configure nat
in the customer router.

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The function of nat is to translate the network
address, in this case nat will translate the

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source address.

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So if the source is 192.168.10.2, after nat
process in router 3, the source address will

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become the public ip address of customer router,
which is 10.20.20.2

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After that, the packet will forwarded to the
ISP, ISP will see the packet,

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now the source address is public ip address, and ISP know
about this ip address, so the packet will

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be forwarded to the internet.

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Okay so that is the concept of network address
translation, I hope you understand.

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I think enough for this video, in the next
video, we are going to configure nat

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so all client will able to communicate with internet
or google.

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Thank you for watching and see you on the
next video .