Each layer of the OSI model and TCP/IP explained.

so first of all the OSI and tcp/ip models application presentation session transport network datalink and physical this is the hierarchical structure of the open systems interconnect model or the open systems interconnection model and and you want a mnemonic to memorize it so the first time I taught this class 17 years ago one of the students said their mnemonic was please do not throw sausage pizza away and 17 years later I haven't forgot it so that's what one of the students came up with please do not throw sausage pizza away so please do not throw sausage

pizza away physical Data Link Network transport session presentation application now do you have to have this memorized for the CCNA absolutely you need to know what the seven layers are now the OSI model was a theoretical model it's never actually implemented it's just conceptual what we actually have is the Internet and the Internet is the tcp/ip protocol suite or the tcp/ip internet suite of protocols it's a whole suite of protocols it's not just TCP and IP it's a whole giant suite and we have only four layers there's the application layer the transport layer the internet

layer and the network access layer we don't talk about numbers if we're talking about numbers everybody thinks OSI so tcp/ip application transport internet and this network access layer this is oftentimes called the link layer the link layer so cisco calls it the network access layer but it's oftentimes referred to as the link layer layer three the network layer and the OSI model is the internet layer in the tcp/ip model I am always good with that why because this is the layer of IP internet I PE this is that layer transports transport that's the super easiest

okay and then the session presentation and application layer are all bundled into one at the application layer it does make sense it makes sense and I'll show you why the reason is at the application layer whether it's application presentation or session or dealing with data here your data has not been broken up into bitsy into pieces yet hasn't been broken up into packets the data gets broken up at layer four it gets turned into segments that's where this is where it gets broken up into pieces at layer 4 the the data is called segments at

layer 3 we call the data packets and layer 2 frames and at layer 1 bits also sometimes at layer 3 they call instead of packets they use the word datagrams why because sometimes we use the term packets like with a big p like it's all broken up into packets so when they say that you wonder hey are they meaning packets like generically or they meaning packets on the third layer right so oftentimes you have to kind of figure that out like sometimes in the reading when they say packets they mean the third layer PDU protocol

data unit the the unit of data that exists at this layer ok devices these layers also coordinate to devices so at layer 1 this is the layer of hubs network interface cards wireless access points but more importantly layer 1 is the layer of the media of the wire it's the layer of the wireless signals the fiber optic wire the ethernet copper cable it's the the layer of the wire however there are devices that function on this layer a wireless access point a NIC a hub all function at layer 1 but a lot of those devices

also function at layer 2 so notice wireless access point NIC at layer 2 a device that's specific to layer 2 would be a switch a bridge is another fancy term for a switch well the switch in the early days was called the bridge why because it didn't have a lot of ports it was just two ports in one out the other like a bridge by a bridge over one and out the other buts multi-port a switch they oftentimes when they first invented switches they called them multi port bridges then you've got layer 3 router a

multi-layer switch which is a switch and a router combined a wireless router that's layer 3 device and then at layers four five six and seven you know layer four firewall layer seven firewall is a fancy firewall that can actually inspect packets at the application layer can actually look at the programs as the packets go across the network so notice how NIC here and here wireless access point here in here so that kind of makes sense like the network access layer sometimes involves both of them the NIC exists at this whole link layer the protocols so

these are the protocols of the Internet Protocol suite at layer one and layer two let's say because we're talking about the Internet Protocol suite so really right we're talking about the network access layer here media access control also MAC addresses are address resolution protocol this is the layers of Ethernet and then there's other things here that you'll need to know about PPP is a LAN protocol DSL is a wind protocol this is a tunneling protocol this is wireless Ethernet this is wired Ethernet Ethernet is the 802 3 specification wireless Ethernet Wi-Fi is the 802 dot

11 specification do you guys see this SONET and SDH this is fiber optic high-speed LAN stuff that's hybrid that's a fiber optic backbone 100 gigs per second not that type of stuff we're gonna learn about CDP and lldp in this course hdl-c this is a serial connection PPP is a serial connection LAN connection LAN connection you guys have to learn our ARP address resolution protocol massively ok ipv4 and ipv6 we're gonna cover ICMP this is ping amongst other types of messages right Internet control message protocol and this is for ipv6 IPSec this is IP security

built into the lower layers this IPSec provides security to packets on the Internet at layers 3 2 & 1 how awesome is that this is routing protocol stuff these are routing protocols you're gonna learn them then you got TCP and UDP this is the transport layer right here this is what my lecture is going to be route that I'm gonna record later tonight on TCP it's gonna be cool then at layers 5 6 & 7 we've got these protocols here these are protocols that the user uses the user uses a web browser is using HTTP

the user puts a domain name into the web browser it uses DNS client the user gets an IP address automatically he's using DHCP client you FTP with the FTP client telnet with a telnet client right there servers for all these two right you have clients and you have servers SSH you SSH in about connection mail SMTP pop is your mailbox IMAP is a mailbox a network time protocol this is simple network management protocols used for managing the network this is transport layer security which used to be called secure socket layer the new term for this

is TLS then we've got BGP and this is border gateway protocol this is the routing protocol of the Internet in between Internet service providers rip is old-school routing protocol and then sip this is used for for voice over IP all right moving on at the application layer what what is the function basically you need to know what each layer does because you're gonna get a question multiple choice let's say on the CCNA you have to say well they'll say you know if the layer does this which one is it is it this one this one

or this one all right well application layer applications protocols and services that interface with the end-users so what do we know these are applications that the end user uses like a web browsers using HTTP what about the presentation layer well the data is formatted it's converted from different formats or converted to different formats and it's encrypted decrypted compressed decompressed and sent or presented to the user and like when I think of the presentation user when I think of the presentation layer I think of mime types right like mime types to show different data types and

emails and stuff like that then the session layer open close and manage a session between end-user application processes so in other words this is stuff that's built in where the application itself can you know basically manage that connection or that that conversation between end users open closed and manage a session between end user application processes so that's built into this upper layers and once again we're talking the application layer in the tcp/ip model but this is application here presentation here session here okay moving on transport layer super facilitates end-to-end communications between multiple applications simultaneously so

this is always confusing because when I see end-to-end communications to the end-user I think it's sometimes they'll say facilitates end-to-end communications for the end-user I think oh application layer no no the transport layer services the upper layer because it provides ports and each port stands for a different application so like port 80 is HTTP the web right port 22 is SSH port 21 is FTP so in ma by putting these ports here at the transport layer you know it facilitates the programs that want to communicate on different ports and it allows for multiplexing or multiple

services to be sent over the wire simultaneously okay at the transport layer we can have reliable service or unreliable end to end data transport and data stream services with TCP UDP or a newer version is SCTP that's newer but mostly we're talking about in this course TCP and UDP TCP is reliable UDP is unreliable so like all of those upper layer protocols like HTTP uses TCP because it needs to be reliable however DHCP does not need to be reliable because it's just you know it's giving out IP addresses and the clients gonna keep asking until

it gets an IP address so UDP use it is so HTT a DHCP uses UDP right DHCP uses UDP it doesn't need that same reliability like SMTP mail or HTTP web all right connection-oriented connectionless communications and data stream services session establishment and termination so this is a lot of things that are provided if you're using TCP it's connection oriented with a three-way handshake if you're using UDP it's connectionless and data stream services session establishment and termination so in other words you start the conversation and end it if it's TCP that type of thing the network

layer what do we do at the network layer also known as the mom this is the name and the network layer it's also known as the internet layer on the tcp/ip model provides host addressing IP this is also called logical addressing chooses the best path to the destination in the network routing or I'm sorry choose the best path to the destination network routing switch packets out of the correct interface forwarding maintain quality of service QoS at the network layer or the internet layer we can set up quality of service so like if you have voice

over IP you need more bandwidth more quality guaranteed bandwidth if you have video you need more bandwidth but if you have an email well I can wait a little bit who cares right but that doesn't work if you have a phone call right then you need more bandwidth so that's what QoS does it prioritizes traffic and those flags are those switches that let you know hey these packets need more priority they're built into this layer all right connectionless and to end networking in other words we're providing end and networking in other words IP addresses can

be networked across the world across the internet and there does not have to be a prior connection it's just its existing without that why because the connection part the three-way handshake and establishing the connection that's handled at the transport layer so IP is known as a best-effort connectionless end-to-end networking protocol all right the data link layer the data link layer is pretty interesting the data link layer is the layer of your Ethernet NIC right the data link layer is the layer of Ethernet it's the layer of your NIC it's the layer of where you have

Ethernet cables and it comes with two sub layers the LLC the logical link control the LLC which is 802 - that's the upper sub layer provides services to the upper layers so the LLC Talk to the network layer the LLC is right here it's the upper part of the data link layer talks to the network layer provides services to the upper layer and then the media access control defines how devices access the medium we're talking here media access control we're talking about carrier sense multiple access with collision detection for Ethernet that's wired and carrier sense

multiple access collision avoidance if it's Wi-Fi all right media access control if it was a token ring network could involve token passing but we don't use token ring networks anymore we use Ethernet networks right Ethernet one that wore a long time ago so when we use Ethernet hosts computers and devices have host addressing at layer two at the data link layer they have MAC addresses so if you want to actually deliver data from my computer to your computer it's going from my MAC address to your MAC address it's getting delivered to a MAC address so

not only do you need I P addresses to navigate over the internet right but when you get to that local area network when you get to that end network you need MAC addresses to deliver it right on the local area network on the Ethernet network right now layer two is not always a local area network at layer 2 we have layer 2 also with wide area networks so if we're on a local area network we're using an Ethernet frame however if we're on a wide area network there's a different type of frame for a layer

to frame in a wide area network so it looks different so for once so for example an Ethernet frame in a local area network or a PPP frame a serial connection on a wide area network maybe not a PPP frame it could be a hdl-c frame or it could be a different type of frame but not necessarily an Ethernet frame if it's a wide area network also the datalink layer provides error checking in other words if there's a corrupted frame why am I using the word frame because that's the PDU at the data link layer

packets at the network layer peat frames at the data link layer so error checking right so if they're bad frames and the this missing data data is corrupted the data link layer will drop the frame or you know it'll see that you know it can error check and say hey this frames bad all right I skip talking about the physical layer because that's the layer of encoding ones and zeros on the wire with electricity or with light pulses if it's a fiber optic or radio waves if it's wireless so I'm not talking about layer 1

the physical layer but you know fine all right another topic in this week is encapsulation and decapsulation in other words when we're encapsulating we're making packets okay when we're decap slating we're taking those packets and putting them back together so we can get the data so d capsulation we're getting--we're want to return the packets back to data encapsulation or making packets so when we encapsulate we move down the model in other words you're using your web browser you're using an application like a web browser HTTP and then boom it goes down here when it hits

the transport layer it gets broken up into pieces and the transport layer header is added on and then the network layer the network layer header and then the data link layer the ethernet frame header and then the physical layer it's turned into ones and zeros and then it goes over the wire as ones and zeros electrical pulses of electricity on the wire when it goes back and reaches its destination it goes back up the model and gets turned back into data and gets you know viewed by the user that requested it or it gets to

the end point so that's how it works so data at the application layer notice at the transport layer it gets broken up into pieces each piece has a transport layer header and then the data header data header data header data so these are the pieces these pieces are called segments then one of these pieces right here it is there's the piece the transport layer header in the data and at the packet ads the network header on there so now we've got source and destination port numbers now in the network header we have source and destination

IP addresses now at the this layer we add on the frame header so now we've got the data right let's say this is the raw data here that we have this is the picture or the email or whatever we have the transport header with the source and destination port numbers the network header with the source and destination IP addresses and then the frame header with the source and destination MAC addresses there's also more information in those headers those are just the most important components the addressing also in the ethernet frame there is a trailer added

and the trailer will have hash in there used for making sure that there's no corruption error checking right and then at the at this layer at the physical layer it gets turned into bits so and this is represents one packet here let's say one packet of ones and zeros and right or you know one at this point when it gets to here we probably this is one frame but anyway the data turns into a segments each segment gets added with the packet the packet gets turned in with a frame and then the frame into the

bits and the headers are added on this is what enables it to go across the internet reach the destination because we need the addressing right does your letter reach a destination without a name and address zip code no it does not