TCP/IP Model vs OSI Model FREE CCNA 200-301 Course 2025
It's really important that you know the TCP/IP model for the real world you'll often hear network engineers saying there's a problem at Layer 1 or perhaps there's an issue at Layer 3 or there's a problem at Layer 7 what does that actually mean you need to learn the TCP/IP model and be aware of the OSI model. Now why do we have these models? This problem unfortunately still exists today here I've got a Pixel phone Samsung phone and an iPhone one of the reasons the OSI model was developed was standardization in this example Pixel phone supports USBC so I could plug that cable in Samsung phone supports USBC but this is an older iPhone it only supports a lightning cable so this cable is proprietary can't be used with these devices this problem used to also exist in networking where vendors would create their own proprietary systems own proprietary protocols and the OSI model allows us to stop vendors making proprietary protocols that don't work with one another and one of the ideas with the OSI model or Open Systems Interconnection Model created by the International Organization for Standardization or ISO was interoperability standardization of interfaces standardization of protocols fortunately more modern example later versions of Apple iPhones have to support USB-C so now we can take the same cable and use it across different vendor devices rather than having to use a proprietary cable for every vendor device now in the same way in networking we have standard interfaces this is an RJ45 connector technically it's got a different name but in the real world people refer to this as an RJ45 connector this is a CAT5e UTP cable unshielded twisted pair cable I'll talk more about cabling in a separate video but what I want you to realize is this cable can connect to a Cisco switch like that i could connect it to a UniFi switch so UniFi switch Cisco switch in the same way same cable could be used with a Netgear switch or same cable could be used with a TPLink router because of standardization this is a standard interface that can be used across multiple vendors in the old days we used to have proprietary protocols proprietary vendor implementations that meant that if you're for instance using a system from one vendor and you wanted to support another vendor you ended up with two separate networks fortunately today we don't have that problem in today's world we use TCP/IP which means that the protocols or languages that we use on the internet are supported across vendor devices cabling is standardized protocols such as TCP or UDP or OSPF are standardized across vendor devices gives us interoperability between different vendors you don't have to worry about buying a specific type of Ethernet cable like you had to do with lightning cables from Apple as an example i mean on the other side of this cable we have USBA here's USBC these are standard interfaces fortunately the EU have forced Apple to move from lightning to USBC in the same way with the OSI model and TCP/IP model we have standard protocols that are supported by multiple vendors in the TCP/IP model we have what are called RFC's or Request For Comments that specify a protocol such as TCP notice how old it is protocols do get updated so you'll see various changes to protocols but notice as an example I could just search for a protocol such as OSPF RFC and here is OSPF v2 RFC is this notice how old this one is 1998 so when we get into the nitty-gritty of the TCP/IP model OSI model don't lose sight of the reason why this was created in the past we had proprietary protocols nightmare from a consumer point of view great from a vendor point of view because they control it they get all the money we could make some really snarky comments about Apple as an example the ISO allows for multi- vendor software and a layered approach for interoperability between devices the idea here is that we split responsibilities some people are responsible for certain parts of the network other people are responsible for other parts of the network so as an example you as a network engineer may work with these kind of cables but you don't physically create the cables you just order them in and you know it's going to be standardized when it arrives you're not going to create your own fiber cables you could do that you could create your own routers you could create your own switches but generally what we do is we split the responsibilities where someone focuses on creating cables someone focuses on for instance creating routers some of us are focused on configuring those devices some of us are focused on writing applications that use a network but we don't understand how networks route traffic from one device to another as an example so this is what the OSI model looks like we have seven layers starting at 1 going to 7 and their names are All People Sleeping Through Networking Don't Pass now that's obviously not what the layers are called but hopefully that helps you remember the various layers here are the actual layers at Layer 1 we have the Physical Layer at Layer 2 the Data Link Layer, Layer 3 Network Layer, Layer 4 Transport Layer then Session at 5, Presentation at 6 and Application at 7. Now I won't bore you going through all the reasons again for the OSI model but the whole idea is to have standardization and interoperability between different vendors and technologies now I'll be talking about the different layers in more detail in a moment but life isn't as simple as this unfortunately it gets a bit more complicated in the past we had competing organizations with different models we had the OSI model and the TCP/IP model in the real world today we use TCP/IP the protocols that we use today are IPv4 and IPv6 and I'll explain where those fit in the TCP/IP model in a moment but in the past there were different protocols we had protocols such as NetBEUI or IPX other types of protocols so it wasn't always clear-cut which protocol would win would IPX win or would TCP/IP win today fortunately we just have to focus on TCP/IP v4 v6 so this is what you need to know in previous versions of the CCNA course they taught the OSI model there was a lot of debate about how practical this model was in the TCP/IP implementation that we use these days the original TCP/IP model had four layers so we only had Link Layer, Internet Layer, Transport Layer and Application Layer only four layers however in the CCNA and in the real world we use the hybrid TCP/IP model this is a five layer TCP/IP model which consists of the Physical Layer at Layer 1 notice how that's similar to the OSI model we have the Data link Layer at Layer 2 similar to the OSI model and then we have the Network Layer at Layer 3 similar to the OSI model again Transport Layer similar to TCP/IP and OSI model and then we have the Application Layer at Layer 5 so rather than just four layers we have two layers the link layer in the TCP/IP model is split into two data link and physical similar to the OSI model now the one that you need to know for the real world is the five layer TCP/IP model Wikipedia shows the different models available out there so just be aware that Arpanet is how the internet was started we had 3 Layers Application, Host to Host and Network Interface and then we had RFC 1122 with 4 Layers and then there are various 5 Layer models as shown here the one that's most famous is Application, Transport, Network, Data Link and Physical and then we have the 7 Layers Application, Presentation, Session, Transport, Network, Data Link and Physical now don't get lost in the details there are 3 Layers the most important one to know is the 5 layer TCP/IP model that's actually what we use in the real world i'll show you using Wireshark how it actually looks on a network now that can be a bit scary the first time you see it so don't worry too much if you don't understand the Wireshark captures right now as I show it to you in this video but you'll learn this as we go through the course you need to know your Layers you need to understand the protocols used in networks so we going to focus on the TCP/IP model where we have Application Transport Network Data Link and Physical Layers. The 3 OSI Layers are combined into a single application layer but and this is where it might get confusing the real world we still often refer to this as Layer 7 so in the real world network engineers will talk about this being a Layer 7 as in there's a Layer 7 problem in other words a problem at the Application Layer even though we only have 5 Layers in the TCP/IP model or 4 Layers in the original TCP/IP model that comes from the fact that this is Layer 7 in the OSI model now don't get hung up about all the different types of models what you need to know for the exam is the five layer TCP/IP model and you need to know which protocols and which devices are used at which layers you'll see references in books and documentation to the OSI model as well as the original RFC 1122 TCP/IP model but for the exam you need to know this hybrid or 5 Layer TCP/IP model that's the one that we're going to focus on from now on now one of the ideas with the OSI model and TCP/IP model is to put protocols at certain layers so as an example we would have IPv4 IPv6 at Layer 3 in the TCP/IP model TCP UDP at Layer 4 Ethernet PPP HDLC which are encapsulations on various technologies at Layer 2 and then we'd have the physical media that we sending traffic across which could be Ethernet Wi-Fi so as an example a fiber cable or a copper cable at Layer 1 and then at Layer 7 we would have protocols such as HTTP, HTTPS, Telnet, FTP, TFTP, a whole bunch of protocols now again a protocol is just a set of conventions on how we communicate so as an example if we're using HTTP we need to agree on the conventions of HTTP how does the protocol actually work again in English we have certain conventions which are different to say other languages so the idea here is we try and put various protocols into certain layers but for the real world just be aware that it doesn't always work as nicely as this so don't get stuck or hung up on trying to put every protocol into a specific layer some people get into fights about this protocol doesn't reside at this layer it resides at a different layer don't get too hung up about that don't get stuck into trying to fit everything neatly into a model like this so as an example a model for a house is just that it's a model it's not going to be exactly the same as the physical implementation the actual house of the model so there will be variations application developers may decide not to follow the rules or the conventions and put their protocols in weird places but for the exam and for CCNA in your mind we are going to try and put most protocols at specific layers and that's what you're going to learn we're also going to be putting devices at specific layers so as an example a hub at Layer 1 switch or bridge at Layer 2 route at Layer 3 TCP UDP again at Layer 4 and our Layer 7 protocols here but before I explain that in more detail and show you with physical devices let's have a look at what each layer does because you're probably wondering you know what what is Layer 1 about Layer 2 about Layer 3 about etc so Physical Layer is probably the easiest one to understand that's the layer that transmits raw bitstreams as electrical optical or radio signals over physical media such as cables or wireless so here's an example i have a fiber optic cable notice the light single mode fiber optic that's different to say multi mode fiber optic so notice how the pattern is different in multi mode fiber versus single mode fiber this is like a solid pattern and this has a dispersed pattern but notice in that definition we are sending bitst streams in other words data across a optical cable as light that's different once again to a copper Ethernet cable that uses electrical signals so electrical signals are sent through copper cables as an example light traverses through the cable allowing communication from one end of the cable to the other or in this example electrical signals are sent by the transmitting party and then are RFC on the other side that's how devices communicate that again is very different to say Wi-Fi the standards for Wi-Fi are going to be very different to optical that again is going to be different to say satellite so here's my Starlink satellite dish so Starlink's transmission to satellites is going to be different to say 5G so the physical implementation of how data is transmitted on 5G is different to Wi-Fi it's going to be different to copper it's going to be different to fiber it's going to be different to satellite communication but the idea is is that we sending data in other words we are sending bits from one party to the other through some kind of media now the data link layer layer two manages physical addressing or MAC addresses framing and error correction between directly connected devices notice the keyword directly connected devices the reason for that is there are different encapsulations and different ways that things work at Layer 2 so as an example it says MAC address there but not all implementations use MAC addresses here I've got an Ethernet network card that uses a MAC address here I've got another Ethernet network card also uses MAC addresses ethernet uses MAC addresses each device in an Ethernet network is identified by its MAC address the same is true on Wi-Fi so the encapsulation or the way information is transmitted on the network depends on the link that's used the physical media used as an example again this is a network interface card that supports fiber so I can connect a fiber cable to the network card data is going to be transmitted across this cable using source and destination MAC addresses that's how devices communicate on Ethernet the same is going to be true for copper Ethernet the Ethernet that you probably very used to so as an example I could connect this copper cable to this network interface card so again just from a Layer 1 point of view obviously the implementation here is different you are not going to use a fiber cable to try and connect to a copper Ethernet network interface card that's not going to work but from an Ethernet point of view it looks the same so if I captured traffic on the fiber cable and looked at the Ethernet frame it would look the same on copper as well as fiber implementation is the same at Layer 2 but at Layer 1 it's different same is true for Wi-Fi if I look at the destination and source MAC address on Wi-Fi it looks the same so if I'm looking at Wi-Fi and Ethernet it's going to look very similar at the higher layers but obviously at Layer 1 it's very different this is using air this is using copper the physical implementation is very different but things change for instance when you go from Ethernet this is an Ethernet port on this router to something and this is an old example i'll show you some modern examples in a moment a serial interface so let's say my network interface card is in my PC and I've got it connected to this router the traffic is going from the PC to the router through Ethernet but when it goes across a serial link serial links don't use MAC addresses so the framing the error correction etc is different on a serial link so if I was sending traffic from a PC using Ethernet and it went across a serial link to another router if I captured traffic on the serial interface it would look different to the Ethernet interface this is using Ethernet framing the way the the frame looks and I'll show you what that looks like in a moment when we run use Packet Tracer but the way it looks on Ethernet is going to be different to the way it looks on serial now in the same way here's a router with some Ethernet ports if I connected my PC to Ethernet on the router traffic that's sent across here is Ethernet but in this example might go across 4G so the Layer 2 implementation is going to be different across 4G versus Ethernet versus an old implementation like a serial interface wi-fi and physical Ethernet look very similar but if you looked at the encapsulation in other words what the frame looks like at Layer 2 here it would look very different to Ethernet now that word frame I'm using on purpose because when we look at the traffic on a network we will talk about bits at Layer 1 because it's binary bits zeros and ones transmitted via light or electrical signals or something else we talk about frames at Layer 2 i'll show you once again what a frame looks like in a moment but we talk about frames at Layer 2 at Layer 3 we talk about packets so a router sends packets at Layer 3 and then at Layer 4 we talk about segments now there's a lot of information here i'm going to repeat myself so don't worry if it's a bit overwhelming the point is is that different devices reside at different layers of the OSI model such as a switch at Layer 2 the traffic sent at Layer 2 is a frame and Layer 2 looks at the physical addressing which could be MAC addresses on Ethernet and on Wi-Fi looks at framing and error correction between directly connected devices now again the reason we say directly connected is if we look at Ethernet here it's between this network card and the router but when the router routes from one media to a different type of media such as serial the communication here is between this router and say the other router connected on the other side of the cable it's got nothing to do with what this PC is sending on the network in the same way if you send traffic through Wi-Fi to your Starlink that's communication between you and Starlink across say Wi-Fi or could be physical Ethernet so I could plug Ethernet in here different type of communication between the routers communicating through the satellite networks versus you using Wi-Fi on stalling forgive my bad drawings now but let's assume that you've got a PC so here's my PC it's sending traffic via Ethernet to a router so this is Ethernet encapsulation used here so as an example it might be Ethernet 2 when it goes from the router to another router this could be a serial link and on this link here we use MAC addresses but on a serial link there are no MAC addresses the encapsulation used here is Ethernet 2 in a lot of cases but the encapsulation here could be PPP so notice it changes every hop between routers the PC on this side could be using Wi-Fi so I'll just draw Wi-Fi like that notice Wi-Fi is different to serial which is different to Ethernet every hop this changes you're not going to use PPP on your Wi-Fi link every link uses a different encapsulation so at Layer 2 this changes every time we go through a router the router actually strips the Layer 2 information and recreates it when it sends the traffic from one interface to another but IPv4 is an example at Layer 3 is end to end so when you communicate through IP you're communicating from one device to another like this it's end to end now you'll learn about NAT and how we can change IP addresses but essentially for the moment think of it as follows you are sending traffic from this PC to this PC IP addresses are used for end to-end communication routing across multiple links Layer 2 is used on a per link basis so at Layer 3 or the network layer we are providing logical addressing which determines the best path through the network at Layer 1 we have physical addressing again on a network interface card the manufacturer burns in the MAC address or media address for the device you can often change that but notice it's burnt into the network card when you buy a network card it has a burntin address so a MAC address associated with the card that you could perhaps change but it has a hardware address identifying this device on the network ip addresses are logical addresses those are addresses that we as an administrator would configure you can change your IP address think about it when you connect to one Wi-Fi network it may use a specific IP address when you connect to a different Wi-Fi network it may use another IP address so in this example I'm connected to a specific Wi-Fi network i'm just going to hide my Wi-Fi networks to keep that private but notice I have a MAC address and an IP address ip address was automatically allocated ip address at the moment is 192.16801 155 i have my subnet mask and my default gateway or router so address was allocated automatically or through dynamic host configuration protocol or DHCP i have a specific IP address if I jump to a different Wi-Fi network the IP address has now changed it's 1921 1681 132 different IP address different default gateway now MAC addresses don't normally change unless you specify an option here like private Wi-Fi address they say here using a private address helps reduce tracking of your phone across different Wi-Fi networks if you use the same MAC address the whole time which is the burnt-in MAC address your phone can be tracked so to help stop tracking of your device as it moves from one network to another what Apple is doing here is automatically changing the MAC address when you move from one Wi-Fi network to another so again the phone has a burnt-in MAC address that's used to connect to the Wi-Fi network same as here you could change this MAC address but by default it'll just use a specific MAC address to identify that device on the network ip addresses will change and can be changed to allow for routing through a network and we're going to spend a lot of time looking at this what we're going to do is configure routers so forgive my drawings that's supposed to be a router which will have a link say to another router which will have a link to another router and say a router on this side to get from router one to router two what's the best path let's assume that this link is 100 megabits per second and this is only 1 megabits per second it's going to make sense to send it across two 100 megabits per second links rather than say 1 and 2 megabits per second link so you through IP addressing as well as running routing protocols can say okay to get from this network to this network I'm going to route this way and IP addressing allows you to configure a specific subnet as it's called or network per interface so every link or every network that you connect to will have a subnet again look at your home network so have a look on your own device here I have a network of 192.168.1.0 I'll talk about subnetting in another video so don't worry too much about that but notice the first three numbers are 192.168.1 which equates
to the 255.255.255 in the subnet mask again don't worry too much about that if you're not sure i'll talk about subnetting in more detail later but the idea is we have a logical addressing that allows us to route from one network to another on this network and I've just jumped now subnet mask is the same notice the network is 192.168.0 now the 255 there is indicating which is the network address so the network address here is 192.168.0 the previous one was 192.168.1 so they are different networks different subnets as we call it allows us to route traffic from one network to another and you'll learn a lot about that in this course but the idea is is that we have a logical addressing you can change your network addressing very easily in your home network you could set it to something else you might not want to use what your router at home has set up by default so this little TPLink router might be using 192.168.1.0 as the network and you could change that it's
a logical address we often don't change our MAC addresses cuz they burnt in but as you saw in the Apple example Apple is changing the MAC address when you go from one network to another for privacy reasons okay next layer Transport Layer so what is theTransport Layer about it supports communication between end devices across a diverse network the two big protocols used at Layer 4 are TCP and UDP transmission control protocol and user datagramgram protocol think of the following with regards to TCP TCP is like a phone call it's going to make sure that the other person is there and make sure that they get the data so when I call you and you answer you typically say hello and then I would say hello so it's kind of like in TCP where we have what's called the three-way handshake we agreeing on certain things before we transmit data if I want to tell you my telephone number as an example so let's say my telephone number is triple 5551234 I would say my telephone number is 555 and you would say 555 to confirm that you got the information then I would say 1 2 3 4 and you would confirm by saying 1 2 3 4 so I'm telling you the information or the data and then you are confirming it back or acknowledging the data it's a connection orientated protocol and make sure that the other party gets the data so transmission control protocol is connection orientated we set up a connection so I phone you you answer the call we acknowledge that by saying hello how are you and we have sort of a conversation first and then I transmit the data to you and you confirm that you got the data or you acknowledge that you got the data user data gram gram protocol UDP is not like that it just sends the data and says good luck so send it i'm not going to confirm that you got it hopefully you did but I don't know if you got it so that's kind of like writing an old fashioned letter so write a letter put it in an envelope and I post it i don't know if you got it maybe you got it maybe you didn't who knows hopefully you did but I have no confirmation that you got the data or the information whereas with TCP you are confirming that you got every part of the information now Layer 7 or application layer or if you want to be precise 5 to7 represents data users encodes and controls the dialogue the whole idea here is that we are setting up for instance a session between an HTTP client and an HTTP server so the client and the server are agreeing on certain parameters and again we'll cover that in a lot more detail as we go through the course you've interacted with applications all the time if using a web browser as an example that's the most common type of application that we use today but it doesn't have to be other applications use protocols such as FTP or TFTP or SSH or Telnet various applications use various protocols for communication at Layer 7 some of those applications make sure that the data doesn't get lost however when we use voice so we're making a Voice over IP call it doesn't make sense to retransmit data that's lost so if it's lost it's lost we don't make sure that the data arrives successfully so voice protocols simply send the traffic across the network and don't make sure that the other party got it now again there's a lot of information here we are going to spend a lot of time going through this information in the course so don't worry too much about it if you're struggling what I want you to understand for the moment is that at Layer 1 it's about bits we are sending bits of data at Layer 2 we've got MAC addresses at Layer 3 we've got logical IP addresses at Layer 4 we've got TCP or UDP and at Layer 7 or 5 to7 we have got applications such as HTTP FTP etc so again Layer 1 we have Ethernet or Wi-Fi it doesn't have to be those two implementations again serial is a different type of implementation we've got 5G various types of implementations at Layer 1 at Layer 2 we've got encapsulations such as Ethernet 2 the most common type of Ethernet that you'll encounter but on a serial link like this you might have had point-to-point protocol or PPP or HDLC different types of encapsulations are used on different type of media fortunately today it's much easier we're going to focus on Ethernet when I started we had to learn all kinds of implementations ATM HDLC PPP and a whole bunch of others which I won't bore you with today it's easy focus on Ethernet at Layer 3 we have IPv4 IPv6 again different protocols were available at those layers you don't have to run IPv4 or IPv6 in the old days we had a protocol called IPX SPX so that was similar in concept to TCP/IP a whole bunch of other protocols existed NetBUIE is another protocol that existed and you'll find that in old versions of Windows as an example but not in modern versions of Windows in the real world today we are focusing fortunately on TCP/IP because remember it's the TCP/IP model that won the OSI model didn't win TCP/IP is what we use today that's what won so we use TCP or UDP at this layer and at Layer 5 to 7 we using HTTPS Telnet FTP TFTP etc lots of protocols exist here so those are just a sample of that
2025-04-04