Full transcript
0:00hello welcome to another lesson from the
0:02first module of my new course on
0:04networking fundamentals
0:06in this lesson we'll be taking a
0:07practical look at the osi model
0:10if you haven't seen the videos for
0:11lesson one i'd highly recommend you
0:12start there
0:13this lesson is simply a direct
0:15continuation of what occurred in lesson
0:16one
0:18that said let's get into discussing the
0:20osi model
0:22the overall purpose of networking as an
0:24industry
0:25is to allow two hosts to share data with
0:28one another
0:29before networking if i want to get data
0:31from this host to this host i'd have to
0:33plug something into this host
0:35walk it over to the other host plug it
0:36into the other host
0:38networking allows us to automate all
0:40that by allowing the host to share data
0:42automatically across the wire
0:45for these hosts to do this they must
0:47follow a set of rules
0:48this is no different than any language
0:51english has a set of rules that two
0:53english speakers must follow
0:54spanish has its own set of rules french
0:56has its own set of rules
0:58while networking also has its own set of
1:00rules
1:01the rules for networking are divided
1:03into seven different layers and those
1:05layers are known as the osi model
1:08now to give you an analogy the human
1:10body is made up of various systems
1:12the skeletal system the respiratory
1:14system the nervous system
1:16the cardiovascular system the muscular
1:18system
1:19and if each of those systems are
1:21operating as intended
1:22then the goal of the human body is
1:24attained meaning the human lives
1:27well networking works the same way each
1:30of the layers of the osi model
1:32serve a specific function and if all
1:35seven
1:35functions are accomplished then the goal
1:37of networking is attained
1:39which is to say host can share data
1:42and the goal of this lesson is not
1:44simply to memorize the osi model
1:46the goal this lesson is to take a look
1:48at the purpose of
1:49each layer in the osi model and see how
1:52it contributes to the overall goal of
1:54networking which is allowing two hosts
1:56to share data with one another
1:58that said let's just jump right into it
2:00starting with the first layer of the osi
2:02model known as the physical layer
2:05data on computers exist in the form of
2:08bits
2:08that is ones and zeros well the first
2:11thing that needs to exist for these
2:12hosts to share a data with another
2:14is something has to transport those ones
2:16and zeros from this computer
2:18to the next that something is the
2:21physical layer
2:22the goal of the physical error is
2:24transporting bits
2:26and anything that contributes to moving
2:27ones and zeros from this computer to
2:29this computer is considered a layer one
2:32technology
2:33for example cables all fall under the
2:36concept of layer 1 technologies
2:38all of these cables serve the purpose of
2:41taking bits from one end
2:42and taking them out to the other end but
2:45don't get too caught up on the word
2:46physical
2:47the osi model was written well before
2:50the idea of
2:51wireless internet communication was ever
2:53thought of
2:54wi-fi is considered to be a layer one
2:56technology because wi-fi
2:58solely exists to carry ones and zeroes
3:00from one computer to the next
3:02now not only do these cables or wi-fi
3:05exist at layer one
3:06there are some devices that also exist
3:08at layer one one of those devices is
3:10known as a repeater
3:12in the last lesson we identified the
3:13repeaters simply amplify signals
3:15from one end out the other end in a way
3:18a repeater is really nothing more than
3:19something that allows you to extend
3:21a wire meaning it contributes to this
3:23goal
3:24therefore a repeater is considered to be
3:26a layer 1 technology
3:28which means that multi-port repeaters
3:31like hubs would also be considered
3:33layer 1 technologies all of these items
3:36serve the function of transporting bits
3:38from one computer to the next
3:40that is the goal of layer 1 which brings
3:43us
3:43to layer 2. layer two is going to
3:46interact with the physical layer it's
3:48going to actually put
3:49bits on the wire and retrieve bits from
3:52the wire
3:54which means whatever this wire actually
3:56connects to on this pc
3:57is considered a layer 2 item meaning the
4:01nic
4:01or the network interface card that this
4:03wire is actually connected to on this
4:04computer
4:05is considered layer 2. the wi-fi access
4:08card is also considered layer 2 because
4:10it's interacting with those wi-fi radio
4:12waves
4:13now nicks and wi-fi access cards don't
4:16really look like this anymore but there
4:17was a time when you had to buy
4:19separate cards and actually install them
4:20into computers and able to access the
4:22network or wi-fi
4:24nowadays wi-fi access cards can fit in
4:26your smartphone or your watch
4:29the overall goal of layer 2 is what i'm
4:32going to call
4:33hop to hop delivery meaning layer 2
4:37exists to take ones and 0s from this nic
4:40and move it to the other nic this
4:43is what i'm calling a hop now to
4:46accomplish this goal
4:47layer 2 is going to use a specific
4:49addressing scheme
4:50that addressing scheme is known as a mac
4:52address
4:53mac addresses are 48 bits which are
4:56represented as 12
4:57hex digits here are three examples of a
5:00mac address
5:01now in all cases these are the exact
5:03same mac address
5:04the only thing that's different is how
5:06they're displayed
5:08windows machines typically use the
5:10dashes in between the hex digits
5:12linux machines typically use those
5:14colons and cisco
5:16routers and switches typically use the
5:18four hex digits with a dot in between
5:20either way it's still just 12 hex digits
5:23which are converted into 48 bits
5:26the way it works is every single nick
5:28has a unique
5:29mac address meaning this computer's nick
5:32over here has the mac address a1
5:34a1 and this computer's nick has the mac
5:36address e8e8
5:39now i'm only showing you the first four
5:41digits of the mac address simply to
5:42conserve
5:43screen real estate but in reality this
5:45is what a mac address looks like
5:47either way this mac address is what's
5:49going to allow data to go
5:51from one nic to the next ie from one hop
5:55to the next
5:58so we've identified that nics exist at
6:01layer two
6:02but there is another type of device that
6:04also exists at layer 2 and that is
6:06switches recall from the last lesson
6:08that we discussed that switches are
6:10devices which facilitate communication
6:11within a network
6:13meaning if these two hosts are connected
6:16via this switch
6:17the switch is what's going to help
6:18traffic move along to accomplish
6:21this hop moreover switches allow you to
6:24connect
6:25many devices to them if these two hosts
6:28want to speak to each other
6:29the switch is going to internally
6:30connect these wires such that data
6:33can traverse this hop from this nick to
6:36this nic
6:38so switches are considered layer 2
6:40technologies because they aid in the
6:42accomplishment of this goal
6:44now generally with the internet you're
6:46commuting with things that are far
6:48away not necessarily things that are in
6:50the same network
6:51so it's very common for communication
6:53between hosts to require multiple hops
6:55meaning we need to jump across multiple
6:57routers to get to the target host
7:00well each of those routers are connected
7:01to a wire using a nic
7:03and therefore each of those nicks have
7:05their own mac addresses
7:06and layer two will handle taking data
7:09from the first mac address
7:10and delivering it to the next mac
7:11address and then from this nic
7:14to the next nic and then from this nick
7:16to the next nick
7:17and then finally from this neck to the
7:19next nick as you'll notice
7:21in all cases layer two is handling the
7:23hop-to-hop
7:24delivery of data which begs the question
7:29if layer 2 is taking care of every hop
7:32what's taking care of ensuring data goes
7:34from this end point
7:36to the next well that's where layer 3
7:38comes into play
7:40layer 3's goal is what's called end to
7:42end delivery
7:44to accomplish its goal player 3 is going
7:46to use its own addressing scheme
7:48known as ip addresses now we unpacked ip
7:51addresses in the prior lesson
7:53ip addresses are 32 bits represented as
7:564 octets
7:56each that can be the number zero through
7:58255.
8:00every host is going to be identified by
8:02its ip address
8:03and these ip addresses are what's going
8:05to allow data to go from here
8:07all the way to here now as you can see
8:10from this illustration
8:12aiding in the goal of delivering data
8:14from end to end
8:15is routers so routers are considered to
8:17exist at layer 3 of the osi model
8:20hosts are also considered to exist at
8:22layer 3 of the osi model
8:24but really anything with an ip address
8:26can be considered to exist at layer 3
8:28of the osi model now at this point
8:32a question typically comes up and that
8:34question is
8:35if we have ip addresses at layer 3 why
8:37do we need mac addresses
8:39or alternatively if we have mac address
8:41to layer 2 why do we need ip addresses
8:44answering that question will help reveal
8:46how packets flow through the internet
8:48the reason we have two different
8:50addressing schemes is that each of those
8:51addressing schemes
8:52serve different purposes so let me show
8:55you
8:56let's say this host has some data that
8:58needs to send
8:59to this host now of course that data is
9:02just a bunch of ones and zeroes
9:04layer two and layer three don't know
9:06what that data contains it's just an
9:08arbitrary set of ones and zeros that
9:10need to get to this host over here
9:13since this computer knows the data needs
9:15to get to this endpoint over here
9:18it's going to add some layer 3
9:19information to that data that layer 3
9:22information is going to include the
9:24source
9:24ip address and the destination ip
9:26address in order to get
9:27this data from one end to the other but
9:30now this computer knows that the first
9:32step will be getting that data to the
9:34first router which will prompt the
9:36computer to add
9:37layer 2 information to that data and
9:40that layer 2 information is going to
9:41have a source mac address of the
9:43computer's nic
9:44and a destination mac address of the
9:46first router's nic
9:47that's what's going to get this
9:49information to the first router
9:52once it gets there we can remove that
9:54layer 2 information
9:56remember the whole purpose of that layer
9:582 header was simply to get
10:00this construct from here to here through
10:03the first hop
10:04from this point this router now knows
10:07that this
10:08needs to get moved to the next router
10:10and it's going to do this by adding
10:11another layer 2 header
10:13but notice this time the source and
10:15destination mac address
10:17identify these two mac addresses that's
10:20what's going to take
10:21this packet to the next router
10:25and here once again we can get rid of
10:27that layer 2 information
10:29again the whole purpose of that layer 2
10:31header was simply to get the packet from
10:33here
10:34to here since it did that successfully
10:36we no longer need it
10:38this process will continue with this
10:40middle router over here
10:41adding layer 2 information that will
10:42take care of this hop
10:47and finally the last router in the
10:49sequence will add the final layer 2
10:51header
10:52which will take the packet from this
10:54nick to
10:55the end host's nick once it gets to the
10:59final host
11:00again the layer 2 header can be removed
11:02the purpose of that header was again
11:04just to bring the packet from here
11:06to here moreover the layer 3 header can
11:10be removed
11:10because the purpose of that header was
11:12to bring the data from
11:14here to here from end to end
11:18finally this gets the data to the
11:20computer and the data can be processed
11:21by the receiving host
11:24that is why we need both ip addresses
11:26and mac addresses
11:27because they serve different functions
11:31and we've been talking about ip
11:32addresses and mac addresses as
11:34independent functions
11:35and indeed they are but i should mention
11:38at this point that there is a protocol
11:40that's going to tie these together
11:42that protocol is known as the address
11:43resolution protocol
11:45it's going to link a layer 3 address
11:47like an ip address
11:48to a layer 2 address like a mac address
11:51arp is crucial to understanding how data
11:53flows through a network
11:55we're going to be talking about arp
11:56later on in this module
11:58but if you can't wait i did write an
12:00article series that discusses all the
12:01different forms of arp
12:03you can access that series at
12:04pracnet.net arp
12:07as for us that wraps up the first part
12:09of our practical discussion of the osi
12:11model
12:12in this video we discussed layer 1 layer
12:152 and layer 3
12:16and how each of those layers have a
12:18specific goal which contributes to how
12:20data flows through a network
12:22in particular we illustrated how layer
12:24two and layer three
12:25work together to move data across the
12:27many different hops
12:29required to get data from one end to
12:31another
12:33so that wraps up part one of our
12:34discussion in part two we'll pick up
12:36right where we left off discussing the
12:38transport layer
12:39but the main takeaways for this lesson
12:41are on your screen right now
12:43i hope you enjoyed this video i want to
12:44thank you for watching and we'll see you
12:46in the next video as we continue our
12:48discussion of the osi model
12:51hey youtube i hope you enjoyed that free
12:53lesson for my new course on networking
12:54fundamentals
12:55i'll be releasing the entire first
12:57module for free here on youtube
12:59i want this course to be the ultimate
13:00networking fundamentals course and since
13:02i'm still scoping out the outline you
13:04could have a say in what topics will be
13:05covered
13:06let me know in the comments below what
13:07subjects you want included in this
13:09course
13:10otherwise remember to like and subscribe
13:12and of course if you learned something
13:13from this video the best way to thank me
13:15is to share this video
13:16it's a small act of gratitude but one i
13:18appreciate greatly
13:20i hope you enjoyed this lesson i want to
13:21thank you for watching and we'll see you
13:23in the next one