To assist us in really appreciating
the benefit of a VLAN, I want to talk for a moment about a layer
2 broadcast.
Let's imagine that PC 1 here is sending a layer 2 broadcast
into this switch network.
Now from our previous discussions,
do you recall what a switch does when it receives a broadcast
in on one of its ports?
What does it do?
And if you'll recall, the answer is the switch
is going to forward that layer 2 broadcast out of all
of its other ports.
So that broadcast coming in on GIG 0/1 on the switch
would be sent out this port, and that port, and that port.
And then when switch 2 receives that broadcast on its GIG 1/0,
it's going to forward it out every other port as well.
So effectively, the broadcast that PC 1 started
is going to be seen by every single host on the network.
Well, if we have 500 or 600 devices,
and we have to see all the broadcasts
from every other device, that is one reason
to help limit the size of a broadcast domain.
And the ability to limit or to cut down
on the size of a layer 2 network is what VLANs do for us.
A VLAN is a layer 2 boundary regarding
how far a broadcast will go, instead of a network
infrastructure.
So in our topology here, if we decided
we wanted these two PCs, for example,
to be in their own virtual local area network,
and we wanted all these devices to be
in a yet different virtual local area network,
we could do that by simply controlling the ports
on the switch.
So we could put port GIG 0/1 and GIG 0/2 in the green VLAN,
and we could put GIG 0/3.
And over here on switch 2, GIG 1/1, and 1/2, and 1/3
all in the red VLAN, and that's how
we control the layer to boundaries
for our VLANs for our broadcast domains.
So if somebody walked up to us and said,
hey quick-- tell me what is a VLAN?
We could say it's a layer 2 broadcast domain.
Ta-daa, that's the full story.
And normally, what happens as well when we start carving up
our networks into virtual local area networks,
not only are we going to change the VLANs that devices are in,
but we're also going to change the layer 3 IP network
that they're in as well.
So if these guys are in the 10 network,
we might need to change all the addressing
on these hosts in the red network
and maybe put them on the 20 network, for example.
But we can save the discussion about IP addressing
and how that works in detail for another Nugget
because the focus here is on VLANs at layer 2
controlling how big a network can be
and doing it by controlling the switch ports.
In this Nugget, we've identified three basic things.
Number one, the acronym of VLAN stands for Virtual Local Area
Network.
Secondly, we identified that a VLAN is effectively
a layer 2 broadcast domain, meaning if our VLAN includes
14 ports and 14 devices, if a broadcast
gets sent in on any one of those ports,
it will only be forwarded to the other ports in that same layer
2 VLAN.
And third, we learned that we can
control which individual hosts are
in which VLANs by controlling the switch
ports that those hosts are connected to.
So if we have 16 ports on a switch,
and we want six of those ports to be
in one VLAN, and the other 10 ports to be in a second VLAN,
that's done by configuring the switch
and the respective switch ports to only
be in those specific VLANs.
I hope this has been informative for you,
and I'd like to thank you for viewing.