I have the opportunity to live in Las Vegas, Nevada.
And one of the sayings I've heard about Las Vegas
is that what happens in Vegas stays in Vegas.
Or now, what happens in Vegas, goes on social media.
In a world of VLANs and Layer 2 broadcast domains,
whatever happens in a broadcast domain
stays in that broadcast domain.
But what about computer?
Like, Computer 1 sitting here needs
to communicate with a computer over at the branch office
out here.
How is that done?
How do we get the traffic out of our local network
and send it across the network to the remote location?
And the answer is we're going to leverage
some additional addressing in the TCP/IP protocol stack
and that is addressing at Layer 3, referred
to as IP addressing.
And when we use that IP addressing,
we have devices on a network that forward packets based
on that Layer 3 IP addressing.
Those devices are referred to as routers.
So here we have router 1.
And that's a router primary purpose in life,
is to look at packets and look at the Layer 3 information,
and if those packets need to be forwarded,
those routers-- to the best of their ability--
will go ahead and forward that packet
to the next device in the path.
So if Computer 1 was trying to reach a PC at the branch
office, the data would need to go here from the computer
to the local router, and then from the local router
and this topology to the firewall,
and then from the firewall to this router,
and then from this router however
it routes out to the remote site.
And that process of forwarding packets
based on that Layer 3 information
is referred to as IP routing.
And although we have separate Nuggets
on the nitty gritty details of IP addressing,
the basic concept of IP addressing
is very similar to streets in a city.
For example, it's very likely that the place that you live,
whether it's a condo, or a house, or apartment
is connected to or on a street.
It has an address.
And all of the other buildings and houses on that same street
have something in common, and that's that common street name.
Well in the world of IP networks, each of our networks
is also going to have a name, very similar to a street name,
except we call it a network address.
So for our discussion, let's imagine
that this is one network here at the corporate office,
and that we've assigned a street name for an IP network address.
And let's imagine that the first three
numbers of this IPv4 address represents the network.
So all the devices here on this network
are going to have an IP address that starts with 10.1.0.
That's like the street name.
And then they'll have individual host numbers or host addresses.
So the printer's at dot 13, Computer 1's at dot 11.
So each of their addresses on this local network
would appear as 10.1.0 dot followed by that last number.
Another network that we have this topology
is this DMZ network up here, and it is network 10.2.0,
for our discussion.
So just imagine the first three numbers
represent the actual network address.
And then over here at the branch office,
we've assigned it the network address of 10.5.0
And we've got some PCs and other devices at that location.
For example, one of our PCs ends in dot 55.
There's a router connected to that network.
It ends in dot 2.
So here's the play by play.
If the user right here at Computer 1
wanted to do a ping request just to verify
that it had connectivity over to the device
at the branch office that was at 10.5.0.55,
the user would get out their command prompt
and they would type in "ping," space "10.5.0.55,"
and press Enter, and just hope for the best.
But behind the scenes is where it really gets interesting.
The local computer says, OK, I'm going
to go ahead put together a ping request that's
destined to the Layer 3 IP address of 10.5.0.55
and it's going to be from me.
My source address-- my IP address--
is 10.1.0.11.
And that goes in the Layer 3 header.
In fact, as a point of convenience,
it might look just like this, the source
address 10.1.0.11 with the destination address
being 10.5.0.55.
But the computer also says this.
Uh oh, I'm on 10.1.0, says PC 1, and the destination
is on a different street-- a different network-- at 10.5.0.
And so at that point, Computer 1, behind the scenes,
realizes that it needs some help.
And if Computer 1 has been configured
to use a default gateway--
and in this topology, a really good default gateway,
the person to go to, like, the Obi-Wan Kenobi
device-- help us, Obi-Wan.
You're our only hope to get out of this network.
In this case, a good default gateway
that Computer 1 should be configured to use
is the router with the IP address of 10.1.0.1 right here.
And if Computer 1 has been configured
to use that, what Computer 1 will do,
when it creates the Layer 2 frame,
it will put its own Layer 2 address as the source MAC
address, the source hardware address, the source Layer 2
address, and it will put the router's Layer 2 address
as the destination at Layer 2.
So here, if we go back to this frame that's been captured,
the source is PC 1 source address,
which is this bad boy right here in black,
and it's putting the destination Layer 2 address of the router
interface right here in the Layer 2 frame, which
is this guy right there.
And then Computer 1 spits that out on the network.
The switches forward that information
based on the Layer 2 address.
That frame arrives at the router, who opens it up, takes
a look at the Layer 3 information, and says,
hmm, this packet needs to be delivered
to the 10.5.0 network.
So if it knows where to forward it,
it would then make a forwarding decision of that packet.
And at Layer 2, it would send the Layer 2 address
of the next hop in the path.
In this case, it would be the firewall.
And then the firewall would get it.
It would also look at the Layer 3 destination address.
And if it knows how to forward it,
it would rewrite the Layer 2 header
and send it to the next router in the path.
And that would repeat until it gets to its destination
network.
And that, my friends, is an overview
of how IP addresses at Layer 3 can
be used by routers to make forwarding
decisions across the network.
I hope this has been informative for you.
And I'd like to thank you for viewing.