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I have the opportunity to
live in Las Vegas, Nevada.

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And one of the sayings
I've heard about Las Vegas

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is that what happens in
Vegas stays in Vegas.

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Or now, what happens in
Vegas, goes on social media.

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In a world of VLANs and
Layer 2 broadcast domains,

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whatever happens in
a broadcast domain

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stays in that broadcast domain.

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But what about computer?

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Like, Computer 1
sitting here needs

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to communicate with a computer
over at the branch office

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out here.

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How is that done?

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How do we get the traffic
out of our local network

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and send it across the network
to the remote location?

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And the answer is
we're going to leverage

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some additional addressing
in the TCP/IP protocol stack

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and that is addressing
at Layer 3, referred

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to as IP addressing.

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And when we use
that IP addressing,

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we have devices on a network
that forward packets based

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on that Layer 3 IP addressing.

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Those devices are
referred to as routers.

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So here we have router 1.

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And that's a router
primary purpose in life,

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is to look at packets and look
at the Layer 3 information,

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and if those packets
need to be forwarded,

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those routers-- to the
best of their ability--

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will go ahead and
forward that packet

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to the next device in the path.

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So if Computer 1 was trying
to reach a PC at the branch

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office, the data would need
to go here from the computer

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to the local router, and
then from the local router

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and this topology
to the firewall,

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and then from the
firewall to this router,

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and then from this
router however

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it routes out to
the remote site.

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And that process of
forwarding packets

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based on that
Layer 3 information

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is referred to as IP routing.

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And although we have
separate Nuggets

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on the nitty gritty
details of IP addressing,

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the basic concept
of IP addressing

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is very similar to
streets in a city.

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For example, it's very likely
that the place that you live,

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whether it's a condo,
or a house, or apartment

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is connected to or on a street.

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It has an address.

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And all of the other buildings
and houses on that same street

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have something in common, and
that's that common street name.

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Well in the world of IP
networks, each of our networks

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is also going to have a name,
very similar to a street name,

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except we call it
a network address.

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So for our discussion,
let's imagine

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that this is one network
here at the corporate office,

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and that we've assigned a street
name for an IP network address.

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And let's imagine
that the first three

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numbers of this IPv4 address
represents the network.

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So all the devices
here on this network

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are going to have an IP address
that starts with 10.1.0.

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That's like the street name.

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And then they'll have individual
host numbers or host addresses.

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So the printer's at dot
13, Computer 1's at dot 11.

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So each of their addresses
on this local network

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would appear as 10.1.0 dot
followed by that last number.

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Another network that
we have this topology

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is this DMZ network up here,
and it is network 10.2.0,

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for our discussion.

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So just imagine the
first three numbers

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represent the actual
network address.

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And then over here
at the branch office,

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we've assigned it the
network address of 10.5.0

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And we've got some PCs and
other devices at that location.

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For example, one of
our PCs ends in dot 55.

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There's a router
connected to that network.

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It ends in dot 2.

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So here's the play by play.

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If the user right
here at Computer 1

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wanted to do a ping
request just to verify

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that it had connectivity
over to the device

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at the branch office
that was at 10.5.0.55,

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the user would get out
their command prompt

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and they would type in
"ping," space "10.5.0.55,"

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and press Enter, and
just hope for the best.

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But behind the scenes is where
it really gets interesting.

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The local computer
says, OK, I'm going

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to go ahead put together
a ping request that's

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destined to the Layer 3
IP address of 10.5.0.55

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and it's going to be from me.

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My source address--
my IP address--

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is 10.1.0.11.

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And that goes in
the Layer 3 header.

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In fact, as a point
of convenience,

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it might look just
like this, the source

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address 10.1.0.11 with
the destination address

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being 10.5.0.55.

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But the computer also says this.

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Uh oh, I'm on 10.1.0, says
PC 1, and the destination

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is on a different street-- a
different network-- at 10.5.0.

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And so at that point,
Computer 1, behind the scenes,

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realizes that it
needs some help.

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And if Computer 1
has been configured

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to use a default gateway--

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and in this topology, a
really good default gateway,

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the person to go to,
like, the Obi-Wan Kenobi

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device-- help us, Obi-Wan.

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You're our only hope to
get out of this network.

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In this case, a
good default gateway

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that Computer 1 should
be configured to use

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is the router with the IP
address of 10.1.0.1 right here.

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And if Computer 1
has been configured

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to use that, what
Computer 1 will do,

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when it creates
the Layer 2 frame,

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it will put its own Layer
2 address as the source MAC

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address, the source hardware
address, the source Layer 2

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address, and it will put
the router's Layer 2 address

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as the destination at Layer 2.

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So here, if we go back to this
frame that's been captured,

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the source is PC
1 source address,

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which is this bad boy
right here in black,

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and it's putting the destination
Layer 2 address of the router

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interface right here in
the Layer 2 frame, which

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is this guy right there.

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And then Computer 1 spits
that out on the network.

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The switches forward
that information

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based on the Layer 2 address.

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That frame arrives at the
router, who opens it up, takes

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a look at the Layer 3
information, and says,

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hmm, this packet
needs to be delivered

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to the 10.5.0 network.

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So if it knows
where to forward it,

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it would then make a forwarding
decision of that packet.

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And at Layer 2, it would
send the Layer 2 address

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of the next hop in the path.

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In this case, it
would be the firewall.

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And then the firewall
would get it.

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It would also look at the
Layer 3 destination address.

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And if it knows
how to forward it,

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it would rewrite
the Layer 2 header

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and send it to the next
router in the path.

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And that would repeat until
it gets to its destination

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network.

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And that, my friends,
is an overview

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of how IP addresses
at Layer 3 can

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be used by routers
to make forwarding

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decisions across the network.

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I hope this has been
informative for you.

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And I'd like to thank
you for viewing.