Hub, Bridge, Switch, Router - Network Devices - Networking Fundamentals - Lesson 1b

hello welcome to the second part of our

lesson on network devices

this is the first lesson from the first

module of my new course on networking

fundamentals

the purpose of this module is to teach

you how data flows through the internet

in part one of this lesson we discussed

the concepts of a host

an ip address and a network if you

haven't watched that video go ahead and

pause this video right now and watch the

first video

there'll be a link in the description in

this video we're simply going to

continue right where we left off

now the main idea we want to teach in

this video are these last two devices

switches and routers but we can't really

understand those

until we understand where we've come

from so we have to start there

in the last video we unpacked the idea

of a network we identified that a

network is created anytime you connect

two computers to each other using a wire

one thing to understand about sending

data across a wire is that it decays as

it travels greater and greater distances

if the two computers you're connecting

are in the same room then you don't

really have to worry about it

the decay will still occur but the

signal will still get through and

therefore

connectivity between these hosts is

still attained if

however these hosts span greater

distances maybe

you're connecting two computers on

opposite sides of a building or even in

two different buildings

then you might have a problem if the

signal decays before it gets the other

side

then these two hosts cannot share data

in those cases what you need is a

repeater a repeater

is a device whose sole purpose is to

regenerate signals

anything that comes in on one end simply

gets regenerated out the other side

this allows you to connect devices

together which span greater distances

so far we've been talking about

networking from the perspective of

connecting one host

directly to another host well if you add

a third host

you now have to connect that host to all

the other hosts which you've already

established and if you add a fourth host

you now have to connect this fourth host

to all the hosts that already exist

and again if you add a fifth host you

now have to connect this fifth host

to every host that has already been

connected as you can see

connecting host directly to each other

simply doesn't scale

instead we created devices which we

could put at the center of every network

and connect all the hosts to those

devices

and these devices would then handle

funneling communication between these

different hosts

the benefit to these types of devices is

that if a sixth host gets spun up

it's very easy to simply connect it once

to that device and now it has

connectivity to every host that has

already existed

that's what all of these are and the

first of these types of device that

we're going to discuss

is known as a hub a hub

is nothing more than a multi-port

repeater

earlier we discussed repeaters and we

said all they do is regenerate signals

hubs do the same thing except they do it

across multiple ports

for example if these two hosts over here

need to communicate

one of them sends a packet to the other

it'll hit the hub and the hub will

simply duplicate that packet and send it

out all remaining ports

that allow what this guy sends to arrive

over here

this fixes the scale problem hub is the

first device that allows us to connect

multiple devices in the center and now

all of them have connectivity to each

other

but as you can probably see the problem

with the hub is that everybody receives

everybody else's data

these two hosts over here which are

uninvolved in the communication between

these two hosts

are receiving a copy of everything they

send

which brings us to bridges here

we have two sets of hosts all

interconnected using a hub

and a bridge is meant to sit in between

hub-connected hosts

bridges by definition only have two

ports one port facing one set of

hub-connected devices and another port

facing the other set of hub connected

devices

bridges will also then learn which hosts

are on which

side of the bridge this would allow the

bridge to contain communication

to only the side that is necessary for

example

if these hosts again need to speak to

each other

when they send data to each other

through that hub the hub is of course

going to simply regenerate that signal

at all ports

and notice that the bridge can be

getting a copy of that packet

but the bridge knows that the other

green host is on this side of the bridge

and therefore the bridge isn't going to

bring that packet to the other side

the bridge is the first type of device

that helps contain packets only to their

relative networks

on the other side if these hosts need to

speak to each other

they can also send packets to each other

through their hub and once again the

bridge will not let those packets bleed

into the other side because it knows

the yellow devices exist on the right

hub

and of course if this device needs to

send something to this device

the bridge is going to know that that

traffic is going to have to cross the

bridge

and the bridge will allow that packet to

traverse to the other side

the main takeaway is understanding that

bridges can learn which hosts are

connected

on either side of the two ports of the

bridge now this

finally brings us to switches switches

are sort of like a combination

of hubs and bridges they are like hubs

in the sense that many devices can

connect to the switch

and they are like bridges in the sense

that they can learn which hosts are

connected to each port

the main difference is that they're

doing it on a per port basis

which means if these two hosts want to

speak to each other

the switch will know that the only ports

that need to receive this traffic

are the two that are connected to those

green hosts and will keep that

communication contained to just

those ports moreover if these two hosts

want to speak to each other

the switch will again make sure that

that communication only flows between

the relative ports

so this is how a switch is like a

combination of a hub and a bridge

the formal definition of a switch that

we want to use is that a switch is a

device

which facilitates communication within a

network

earlier we defined a network as a

logical grouping of hosts which require

similar connectivity which means

all of these devices over here all

belong to the same

network moreover networks

all share the same ip address space

which means this network owns all the ip

addresses which start with

192.168.1.anything

and this host's identity is the ip

address 192.168.1

dot and this host would be 192.168.1.66

and this set of devices could very

easily represent

all the different hosts on your home

wi-fi network

maybe this device is your printer and

this device is your laptop

and this device is your mobile phone and

so on

or maybe this network and these devices

represent all the pcs that might exist

within a particular classroom

of the school network or maybe even

further all these devices represent

hosts that exist in the sales team

of the london office of the acme

corporation

one way or another since all these

devices are connected with a switch

they all belong to the same network now

let's go back to that example of the

school network

we said that the school likely has many

different classrooms

and each of those classrooms belong to

their own network

which means this would be a more

accurate representation of the school

network we would have classroom two

owning that ipspace and classroom three

owning that ipspace

now the reason you might want to

separate these two sets of devices into

their own network is because they might

have different connectivity requirements

for example maybe these computers over

here

all belong to the biology classroom and

all they need is simple internet

connectivity

but maybe these computers over here

belong to the computer science classroom

and they not only need internet

connectivity but also access to various

cloud resources

to do their studies well since these

computers have different connectivity

requirements than

these computers it's a good idea to

separate those out into separate

networks

now in both cases we can still use

switches to facilitate

all the communication within the

networks meaning

this switch can handle all the

communication between these three

hosts and this switch can handle all the

communication

between these three hosts

but what happens if this host down here

wants to speak

to this host on a different network

well if a switch can only facilitate

communication within

a network we would need another type of

device to handle the communication

between networks and that device would

be

a router a router is a device whose

primary purpose is to facilitate

communication

between networks at the very least

you're going to need that router to

connect you with the ultimate network of

networks

known as the internet so let's unpack

this further

routers provide traffic control points

between

networks let's say we wanted to limit

the traffic that could go from this pc

to this pc

well since these two pcs aren't separate

networks all that traffic

has to flow through the router creating

a great place

to add security policies or traffic

filtering or even redirecting that

traffic elsewhere entirely

since routers sit on the boundary

between networks

they provide a logical location to apply

security policies

this type of security filtering isn't

traditionally available on switches

these days there are modern switches

that can do such filtering

but it is generally accepted that the

devices sitting on the same network

don't typically need filtering for

traffic traveling within the network

if you had devices that needed different

types of connectivity you'd want to

place them in different networks

the network boundary is what is meant to

be the logical separation of devices

the way routers work is that they learn

which networks that they are

attached to meaning this router is going

to learn that on this interface it's

connected to the 172 16.20 network

and on this interface it's connected to

the 172 1630 network

and out here is the direction to go to

the internet

the knowledge of each of these different

networks is known as a route

and all these routes are stored in what

the router calls a routing

table a routing table is therefore

all the networks that a router knows

about and the router is going to use

this routing table in order to funnel

traffic

out the appropriate interface now

when we say a router learns which

networks they are attached to

what we mean is that a router has an ip

address

in every network that they're attached

to for example

when this router is attached to this

network it is given

an ip address in that network this

interface's identity

is the ip address 172.16.20.1

and this interface's identity is the ip

address 172

16.30.254

this ip address is going to serve as

what's known as a gateway

a gateway is a host's way out of their

local network

for example this host over here has the

ip address 172

16.20.33 but if that host

wants to speak to something on a

different network it knows it's going to

have to go

through a router and the ip address for

that router is stored as that host's

default gateway

notice this host has a default gateway

of 172.16.20.1

that's this interface ip address of that

router

now if we go a step higher than that

routers are actually what create

the hierarchy in networks and ip

addresses that we discussed in the prior

sections of this lesson for example the

new york office of the acme corporation

that had all the different teams that

each had their own ip networks

well each of those networks are

connected to different routers

and each of those routers are then

connected to another router

and if a host in the sales team wants to

speak to a host on the marketing team

it's going to use its gateway which is

its closest router ip address

which is then going to send the packet

to the next router to the next router

and then finally to the host on the

marketing team

the tokyo office of the acme corporation

is likely going to have a similar setup

and both of these routers are then

likely going to connect to the internet

the internet is nothing more than a

bunch of different routers itself

meaning if a host on the marketing team

wants to speak to a host on the

engineering team in tokyo

that host will send the data to the

router which will send the data to the

next router

which will send it through all the

routers on the internet which will

finally send it to the tokyo router and

finally to the engineering team

that is how data is going to flow across

the internet

and that is the role that routers play

in making that possible

now the last idea i want to leave you

with actually involves pulling back the

definition of switches as well

there's something important you have to

understand about what we've defined as

routers and switches

route ding is the process of moving data

between

networks a router as we have described

it

is simply a device whose primary purpose

is to perform

routing in the same way switching

is the process of moving data within

networks

and a switch as we have described it is

a device whose primary purpose

is switching the reason i bring that up

is there are many other types of network

devices that exist out there

access points firewalls load balancers

layer 3 switches proxies

and there's even devices that only exist

in the cloud like virtual switches and

virtual routers

one way or another all these devices are

going to perform

routing or switching or both so later on

in this module when we describe

what a router does or what a switch does

what we are actually describing is what

any device does that implements routing

or any device does that implement

switching

and with that we close our lesson on

network devices

in part one of this lesson we unpacked

hosts

ip addresses and networks and in part

two

we continued that discussion by

illustrating repeaters

hubs bridges switches and routers

in the next lesson we're going to give

you a practical perspective on the osi

model

this will lay the foundation to

understand what all of these devices do

to enable data flowing through the

internet

but that wraps up this lesson your main

takeaways are on the slide right now

i hope you enjoyed this lesson i want to

thank you for watching

and we'll see you in the next one

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fundamentals

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i hope you enjoyed this lesson i want to

thank you for watching and we'll see you

in the next one