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Understanding the OSI Model - CompTIA Network+ N10-009 - 1.1 — Transcript

by Professor Messer · 2,211 words · 324 segments · language en · Watch on YouTube

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0:02we'll start our Network plus course with
0:04a broad discussion of something we refer
0:06to as the OSI model this is referring to
0:09the open Systems interconnection
0:12reference model and it's a model we use
0:14in it to describe the process that data
0:17takes as it traverses our networks The
0:20OSI model is not designed to be a
0:22detailed description of this data but
0:25instead is intended to describe a broad
0:27overview of how data Travers our systems
0:31we're also not describing the OSI
0:34protocol Suite indeed most of the
0:36protocols we use today are based on
0:39tcpip but the OSI model is one that we
0:42can apply to many different protocols
0:44and it works perfectly with the tcpip
0:46protocols that we use today also keep in
0:49mind that there are many different
0:50protocols that might operate at an
0:53individual layer of the OSI model so as
0:56we step through an individual layer keep
0:58in mind there might be tens or even
0:59hundreds hundreds of protocols that
1:01might exist and work at that particular
1:04layer by understanding this OSI model we
1:07can converse with other people in
1:08information Technologies in a way that
1:11we would all understand it doesn't
1:13matter if they're working inside of your
1:14own company or with a different
1:16organization When you mention OSI layer
1:197 or OSI layer 4 everyone understands
1:23what that means and what effect it has
1:25on the overall operation of the network
1:28if we start at the top of of the OSI
1:30layer or OSI layer 7 and work our way
1:33down to OSI layer 1 we have layer 7
1:36being the application layer layer six is
1:38the presentation layer layer five is the
1:41session layer layer four is transport
1:43Layer Three is Network Layer Two is the
1:46data link layer and layer one is the
1:48physical layer there are many different
1:50pneumonics that people use to memorize
1:53this but if you start at the top and
1:55work your way down you can take the
1:57first letter of each one of these layers
2:00and you might have a term such as all
2:02people seem to need data processing
2:05that's application presentation session
2:07transport network data link and
2:11physical let's start our conversation of
2:13the OSI model at the bottom of the OSI
2:16model or layer one we refer to this as
2:19the physical layer because it's
2:21describing the physical signals that we
2:23send through the cable and fibers on our
2:26Network at this layer we really don't
2:28have many protocols to speak of because
2:31we're really just talking about getting
2:32a signal from one part of the network to
2:35another when we refer to a physical
2:38layer problem with the network we are
2:40referring to the OSI layer one physical
2:43layer this means that we might have a
2:45bad cable a bad fiber there might be
2:47interference on a wireless network and
2:50it may require us to do a series of
2:52tests to determine if we are really able
2:54to get a signal across this particular
2:57wire or this particular fiber so that we
2:59can keep this network running a lot of
3:01the troubleshooting that you do at layer
3:03one is running loot back tests testing
3:06cables and fibers and checking different
3:08adapter cards and other devices to make
3:11sure they're working properly on the
3:13network One Step Up from the physical
3:15layer is Layer Two the data link layer
3:18this is the fundamental layer that's
3:20used to communicate between two devices
3:22on the network we often refer to this as
3:25the MAC address layer because that is
3:27the data link control layer or DLC layer
3:31that is commonly associated with the
3:32network cards that are in our devices
3:35and most of the time these are ethernet
3:37adapters or wireless adapters and we
3:40refer to that physical address on that
3:42device as the data link control address
3:45or the MAC address in this case Mac
3:47address is not referring to an operating
3:50system it's referring to the media
3:52Access Control address so anytime you
3:54hear someone talk about a Mac address
3:56they're referring to the hardware
3:58address of that particular adapter card
4:01since the network switches that we use
4:03on our Network determine how to forward
4:05traffic based on the destination Mac
4:07address this is a layer that we often
4:10refer to as the switching layer so
4:12anytime we're referring to a MAC address
4:15a problem with a switch being able to
4:17operate or anything else that may be
4:19associated with this particular Hardware
4:22address we're referring to OSI layer 2
4:25the data link layer the next layer up is
4:28OSI layer 3 or or the network layer we
4:31often refer to this as the routing layer
4:33because this is the layer that routers
4:35use to determine how to forward traffic
4:38and they are specifically looking at the
4:40destination IP address in order to
4:43determine what the next hop might be for
4:45traffic traversing the network this is
4:48also the layer that we're able to
4:50fragment these frames into multiple
4:52pieces especially if we're sending it
4:54across a network that may require
4:56smaller frames than what is on our local
4:59network so we can cut those frames up
5:01into smaller pieces to be able to fit
5:03them through the network and then put
5:05those pieces back together on the other
5:07side anytime we're referring to a
5:09problem relating to IP addressing subnet
5:12masks anything related to an IP address
5:15or anything about routing then we're
5:17probably referring to layer three the
5:19network layer layer four is the
5:22transport layer and as the name implies
5:25we're referring to the ability to
5:26transport information from one device to
5:29another
5:30you might also refer to this as the post
5:32office layer because this is responsible
5:35for getting your letter or your
5:36information from one side of the network
5:39to the other the protocols that are
5:41often used and operate at layer four of
5:43the OSI model is TCP that stands for
5:46transmission control protocol and UDP or
5:50user datagram protocol these two
5:53Protocols are commonly responsible for
5:55getting all of the information within
5:57our IP packets from one device to the
6:00other in many cases this involves taking
6:02a large amount of data putting it into
6:05smaller pieces to be able to get it
6:07across the network and then putting
6:08those pieces back together on the other
6:11side before we can send that information
6:14from one side of the network to the
6:15other we may need to create a session so
6:18that a device is able to receive that
6:20data layer five is the session layer and
6:23it provides communication management
6:25between point a and point B anything
6:28relating to the initiation of a session
6:30stopping the session or restarting the
6:32session can commonly be associated with
6:35that layer five session layer
6:37communication if an application is using
6:39some type of control protocol or you're
6:41tunneling information within existing
6:44data then you're probably using OSI
6:46layer 5 OSI layer 6 is responsible for
6:50putting all of this data into a format
6:53that we will eventually see with our
6:55human eyes this refers to character
6:57encoding application cryption and
7:00decryption and it's often combined and
7:02discussed in conjunction with the
7:05application layer at layer 7 layer six
7:08or the presentation layer is the layer
7:10that is commonly in operation just prior
7:13to us seeing this data on our screen and
7:16the top layer of the OSI model is OSI
7:18layer 7 or the application layer this is
7:21the layer that we see on our screen so
7:24anytime that we are interacting with an
7:26application we are operating at layer 7
7:29of the OSI model common applications
7:32that would operate at OSI layer 7 are
7:35HTTP and https FTP DNS POP 3 and
7:39thousands of other application protocols
7:42well that's a broad overview of the OSI
7:45model but how do we fit things in the
7:47real world into each of those layers
7:51let's start down at the bottom with
7:52layer one or the physical layer when we
7:55refer to cables fiber optic connections
7:58the signal going across cross those
8:00connections or a wireless network then
8:02we're operating at OSI layer 1 or the
8:05physical layer when we refer to OSI
8:07layer 2 or the data link layer then
8:09we're talking about ethernet frames Mac
8:12addresses or addresses referred to as
8:14extended unique identifiers or eui there
8:18are eui 48 addresses and eui 64
8:22addresses for example and as we
8:24described earlier anytime we're
8:25referring to the switching process we
8:27are referring to OSI later ler 2 at OSI
8:31layer 3 or the network layer we're
8:33referring to IP addresses so anything
8:35that has an IP address or subnet mask or
8:38referring to the way that routers
8:39forward traffic all happen at OSI layer
8:433 if you're referring to a TCP port or a
8:46UDP Port then we're operating at OSI
8:49layer 4 which is the transport layer at
8:52OSI layer 5 or the session layer we're
8:55using control protocols to start and end
8:57different sessions or we may be
8:59tunneling traffic using protocols that
9:01operate at OSI layer 5 we often
9:04associate the encryption of application
9:06data with OSI layer 6 or the
9:09presentation layer so if you're
9:11communicating to a website and you're
9:12using SSL or TLS to be able to encrypt
9:16and decrypt that data that process is
9:18occurring at OSI layer 6 and at OSI
9:22layer 7 or the application layer is
9:24where you are interacting with the
9:26application itself anything that you're
9:28seeing on the screen the application
9:30itself or any messages from the
9:32application are often referred to as
9:34layer 7 information if you wanted to
9:37drill down a little bit further into the
9:40OSI model you might want to capture some
9:42data on your network and view that data
9:45inside of an application such as
9:47Wireshark I have a wire shark protocol
9:49decode on my screen where I've captured
9:51data on my network and this wi shark has
9:54three separate windows that is
9:56displaying different views of the same
9:58data the top window is a frame byf frame
10:01breakdown for example I have highlighted
10:03frame 88 on this top window the middle
10:06window describes more detail of what
10:09different information is contained
10:10within that single frame and on the
10:12bottom window we have a heximal and asy
10:15breakdown of the data itself let's focus
10:19our view on this middle window which is
10:21a summary of this highlighted frame 88
10:24you can see that there are 1 two 3 four
10:27five different lines of data inside of
10:30the single frame where the protocol
10:32decode has separated out the individual
10:34protocols within the single ethernet
10:36frame let's start with the one at the
10:39very top which describes frame 88 which
10:42has 2005 bytes on the wire and 2005
10:46bytes were captured by wire shark if you
10:48wanted to associate that line with the
10:51OSI model that would be best associated
10:53with layer one or the physical model
10:56next down is ethernet 2 which has a
10:59source Mac address and a destination Mac
11:01address and since we are describing Mac
11:04addresses everything within that line of
11:06the wire shark decode is associated with
11:09layer two of The OSI model or the data
11:11link layer the next line down describes
11:14the Internet Protocol or IP and as we
11:17described before IP fits squarely in
11:19layer three of the OSI model or the
11:22network layer and in fact there are
11:24individual IP addresses listed here for
11:26the source IP address and the
11:28destination I IP address and you can see
11:30there is a DNS resolution here that
11:32shows that the IP address of
11:587242431072 the transport layer and
12:00indeed you can see TCP port numbers for
12:03the source and the destination of this
12:05traffic since the destination Port is
12:07Port 443 wi shark has already filled in
12:10that we're communicating over an
12:11encrypted Channel using
12:14https and if we go one layer down into
12:17the secure socket layer this effectively
12:19encapsulates the top three layers of the
12:22OSI model so secure socket layer and
12:25everything underneath it are referring
12:27to layer five the session layer layer
12:29six presentation layer and layer 7even
12:32the application layer here's a summary
12:35of that protocol decode broken out into
12:38the different layers of the OSI model we
12:41had electrical signals at the physical
12:43layer Layer Two at the data link layer
12:45had ethernet frame information our IP
12:48addresses were at layer three or the
12:50network layer our TCP encapsulation with
12:53our TCP port numbers is at the transport
12:56layer our session layer linked the
12:58presentation information to the
12:59transport presentation layer was
13:02obviously performing that SSL or TLS
13:04encryption and decryption and layer 7 or
13:07the application layer is referring to
13:09that Google Mail front end and being
13:12able to send and receive emails using
13:14mail.google.com
13:16hopefully you're now able to see how
13:18this very broad description of an OSI
13:21model can be applied to practically any
13:23communication that's occurring over the
13:25network and this allows us to talk with
13:28other IT professional
13:29and communicate in a way where everyone
13:32understands exactly what we're referring
13:34to when we talk about a layer one
13:36physical problem or a layer 7
13:39application problem

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