1
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To assist us in
really appreciating

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the benefit of a VLAN, I want to
talk for a moment about a layer

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2 broadcast.

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Let's imagine that PC 1 here
is sending a layer 2 broadcast

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into this switch network.

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Now from our
previous discussions,

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do you recall what a switch does
when it receives a broadcast

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in on one of its ports?

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What does it do?

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And if you'll recall,
the answer is the switch

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is going to forward that
layer 2 broadcast out of all

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of its other ports.

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So that broadcast coming
in on GIG 0/1 on the switch

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would be sent out this port,
and that port, and that port.

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And then when switch 2 receives
that broadcast on its GIG 1/0,

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it's going to forward it out
every other port as well.

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So effectively, the
broadcast that PC 1 started

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is going to be seen by every
single host on the network.

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Well, if we have
500 or 600 devices,

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and we have to see
all the broadcasts

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from every other device,
that is one reason

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to help limit the size
of a broadcast domain.

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And the ability to
limit or to cut down

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on the size of a layer 2
network is what VLANs do for us.

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A VLAN is a layer 2
boundary regarding

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how far a broadcast will
go, instead of a network

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infrastructure.

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So in our topology
here, if we decided

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we wanted these two
PCs, for example,

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to be in their own virtual
local area network,

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and we wanted all
these devices to be

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in a yet different virtual
local area network,

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we could do that by simply
controlling the ports

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on the switch.

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So we could put port GIG 0/1
and GIG 0/2 in the green VLAN,

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and we could put GIG 0/3.

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And over here on switch 2,
GIG 1/1, and 1/2, and 1/3

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all in the red
VLAN, and that's how

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we control the
layer to boundaries

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for our VLANs for our
broadcast domains.

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So if somebody walked
up to us and said,

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hey quick-- tell
me what is a VLAN?

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We could say it's a
layer 2 broadcast domain.

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Ta-daa, that's the full story.

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And normally, what happens as
well when we start carving up

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our networks into virtual
local area networks,

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not only are we going to change
the VLANs that devices are in,

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but we're also going to
change the layer 3 IP network

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that they're in as well.

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So if these guys are
in the 10 network,

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we might need to change
all the addressing

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on these hosts in
the red network

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and maybe put them on the
20 network, for example.

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But we can save the
discussion about IP addressing

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and how that works in
detail for another Nugget

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because the focus here
is on VLANs at layer 2

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controlling how big
a network can be

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and doing it by controlling
the switch ports.

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In this Nugget, we've
identified three basic things.

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Number one, the acronym of VLAN
stands for Virtual Local Area

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Network.

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Secondly, we identified
that a VLAN is effectively

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a layer 2 broadcast domain,
meaning if our VLAN includes

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14 ports and 14
devices, if a broadcast

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gets sent in on any
one of those ports,

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it will only be forwarded to the
other ports in that same layer

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2 VLAN.

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And third, we
learned that we can

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control which
individual hosts are

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in which VLANs by
controlling the switch

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ports that those hosts
are connected to.

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So if we have 16
ports on a switch,

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and we want six of
those ports to be

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in one VLAN, and the other 10
ports to be in a second VLAN,

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that's done by
configuring the switch

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and the respective
switch ports to only

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be in those specific VLANs.

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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.

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