>> With our next section, Network Connectivity Devices,
we're going to talk about devices that help us join
systems together so that way we can communicate.
I'm going to start out with hubs.
I'm going to start out the way things
used to be because I really do think
it helps us understand the benefit of
the devices that we're using today.
Up at the top where you see layer 1
that refers back to the OSI model.
The first device we're going to talk about is a hub.
When I came into computing back in the '90s,
hubs were very popular.
They were a good, quick, easy way to
connect a bunch of computers, and that was it.
That was the benefit, that they were cheap and
easy and they would provide connectivity.
As a matter of fact, when I came in,
the hubs weren't even powered.
They were just metal racks that you've plugged
into to provide a conduit for the signal.
Obviously, the hub doesn't have any intelligence.
It doesn't direct traffic or segment the network.
Hubs simply sends all the data
out all the ports all the time.
If computer A has data to send to computer B,
that traffic goes out all the ports
so it's available to computer C and D,
and J and K, and
any other devices that might be
>> plugged into the network.
>> If you can think about that.
If I had a sniffer plugged into a hub,
that would just be a bonus because I would
have access to everything on the network.
That's one of the many reasons we don't use hubs today.
Because from a security perspective,
that can be very dangerous.
One thing I'll mention is that when the data
goes out all parts to all hosts,
the network card examines the data frame,
and what it's looking for is a destination
MAC address that is its own.
When B looks at data,
it looks at the MAC address and says, oh,
that's for me and pulls it off the network,
or that isn't for me and leaves it alone.
All a sniffer does examine all packets the same way,
all frames the same way,
but its interface is in a mode called promiscuous mode,
which means the sniffer doesn't care
who the MAC address is for,
and it doesn't care about destination address.
The sniffer simply pulls all traffic off
the network regardless of the destination.
Promiscuous mode sounds like it should
be a lot more fun than that.
But all it means is the network card is going to
pull traffic regardless of the destination.
With the security issues of hubs and
all data out all the ports all the time,
there's no directory in traffic or help for collisions.
In a hub environment, we have a lot of collisions.
We have all data going out of all
>> the ports to everybody.
>> We have what's referred to as one big collision domain.
A switch is going to fix that
>> problem for us because one
>> of the first things it does is
isolate traffic in the collision domains.
Each port on a switch is its own collision domain.
If we go back to the hub,
every host in this illustration
is part of the same collision domain,
which means they're all competing
>> for time on the cable.
>> When we replace those hubs as switches,
each host has its own collision domain.
Which basically means computer A is just
competing with itself per time on the cable.
We've all eliminated the collisions in
our ethernet network just by bringing switches in.
Another thing that a switch does that helps
us out a lot is direct traffic.
A switch learns the network over time and
learns which hosts out which part using MAC addresses.
If you remember our OSI model discussion,
we said switches were layer two devices.
MAC addressing is layer two.
The switch uses a MAC address, sends out the data,
and learns that the data is picked
up on port 3 by computer B.
It takes B's MAC address and
loads it into a table called the CAM table.
Ultimately, it keeps track of
MAC addresses and their matched port.
Like a police officer at a busy intersection
directing traffic out of
the appropriate port when power is out.
If you think about that, going back to
our discussion about having a sniffer plugged in,
if we plug a sniffer into port 2,
no traffic's going to be directed to port
2 because nobody is sending traffic to the sniffer.
One of the ways that we mitigate against
sniffing the network is to use switches.
However, sometimes a network admin
wants to sniff their own network.
You want to see what type of traffic is going around,
what's being sent with passwords in plain text,
or how much broadcast traffic there is.
In that case, I plug a sniffer into the switch and
enable administrative mode called port SPAN.
What port SPAN allows is all traffic to be mirrored out
that particular port so I can inspect
it. We've got our switches.
Down at the bottom, I have a little asterisk
that I mentioned, bridges.
Bridges were predecessor switches.
Bridges are also layer two devices.
They were used to connect to things like
token ring network to an ethernet network,
but they still provided the use of
isolation and collision domains.
They're the precursor to what we
have today and what we know today.
Switches. When we had
our nice little environment with switches,
one problem we didn't solve was
broadcast traffic. Here's a network.
Let's say I have the salespeople.
The salespeople are over here to the left,
they're computer J and K.
The salespeople have an application
that generates a lot of broadcast.
If we go back to the previous slide,
any system that has a broadcast in this environment,
that broadcast goes to the entire network.
That's what a broadcast does,
goes out to everybody on the entire network.
In our illustration here,
I only have two computers that need that
broadcasts from the sales application.
But it's going to everybody, too much traffic.
The more broadcasts that people don't need,
the more the network gets bogged down.
A router can be brought in to
>> isolate broadcast traffic.
>> Maybe I want the sales network subnetted from
the rest of the network to control broadcast traffic.
In the middle, I have a group of
computers from the Human Resources Group.
They have very sensitive data.
I want to segment that network
so I can apply some security.
Maybe enforced IP set,
maybe be very strict on who accesses
those network systems so I create
a segment for them and they're on their own subnet.
Then with VOIP. Quality of
service is really important for VOIP.
By that being able to prioritize
traffics that the VOIP network
gets all the bandwidth that it needs.
That'll be another reason to subnet
a network, quality of service.
Basically, what I needed to do is subnet
my network out based on either broadcast traffic,
security needs, quality of service,
or maybe just based on logical connectivity.
It makes sense to group a certain group of computers in
the same network just based on access and location.
A router can do those services for me.
A router is a layer three device.
Not only can it segment the network
into different subnets,
but the different subnets can communicate it.
Even though this illustration looks and works great,
the problem is that routers are expensive.
I don't necessarily mean when you look at
your receipt is going to be that
much higher than a switch,
but with a switch, you're going to get lots of ports.
When you purchase a router,
you're going to get one land port.
The routers are primarily used today to get off
your local area network so you get a single LAN port.
We've got to find a way to do is that we
have to have in this picture, but make it cheaper.
That's what's coming up next.