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