Hosts ~ above a network require to interact with every other. This is not as an easy as it sounds. That requires different parts the hardware and software work-related together. They must speak the exact same ‘language’.

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The different languages that network hosts speak are referred to as protocols. Protocols room a defined set of communication rules, the are constructed into network software and hardware.

An example of network hardware is the Network user interface Card or NIC. An instance of network software program is the NIC driver.



A single host will use countless different protocols to accomplish different tasks. Usually, several protocols and systems are provided together. An example of this is once you send an email. The email customer and server interact with a protocol called SMTP. But SMTP only describes how letter works. They also use protocols like TCP come make certain that the post arrives, IP addresses to recognize where come send the message, and also Ethernet to transform the post to electrical signals.

It have the right to be tricky to understand how all these components fit together. To assist with that, we deserve to use the OSI model. This is a theoretical design that division the network components into layers. Friend may have actually heard that terms like ‘layer-3 switch’. The ‘layer 3’ component is introduce to the great in the OSI model.

Each great represents component of the network top top a host. The ridge starts in ~ the bottom v the physical layer and goes best up to the applications layer. Every of the 7 layers communicates through the layer above and the layer below.

The advantage of this is the an application, up in layer 7, does not need to understand anything about how the physics network. The application just does that job and passes information to the great below. The very same is true in reverse. A NIC will obtain bits that information, however it doesn’t care at all around the applications that offers the data. That just concentrates on receiving the bits and passing them increase the stack.

As we look deeper, friend will check out that us don’t usage the OSI design to describe specific protocols. It’s a technique of showing how information in the network is handled, independant the the certain protocols in use.

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How the Model Works

The OSI model consists of seven layers. Below is a rapid overview from the optimal down.

(7) Application – The components of the network stack that communicates with the application. This consists of network APIs

(6) Presentation – This layer converts the data between the application and the network

(5) Session – Builds and maintains a session between application processes

(4) Transport – takes a currently of data, and breaks it into segments. This allows multiplexing top top the network (allowing more than one traffic circulation at once)

(3) Network – gives addressing and routing services

(2) Data Link – Handles shipment of data between two nodes linked to a physics medium

(1) Physical – Transmits and also receives data throughout a physics medium. This might be electrical, light, or radio signals

That’s a lot to remember! If that helps, try to usage a mnemonic to remember the layers. Here’s one that you have the right to use:


Please perform Not litter Sausage Pizza Away

It’s a little bit silly, but that’s what provides it memorable.


You might notification in the diagram on the right, the the layers are grouped into Lower Layers and Upper Layers. The reduced layers (layers 1-4) are where most of the network contents live, and where network designers traditionally spend many of their time. The top layers (layers 5-6) are largely application related, and where developers and application experts traditionally spend more time.


Data Flow

When one application needs to accessibility the network, it starts in ~ layer 7. Data is pass down with the layers toward layer 1, v each great performing important functions. As soon as data is received by a host, that is got at layer-1 and works its method back approximately layer 7, whereby the applications is waiting.

When details is relocating down the layers, some layers will encapsulate the data. Encapsulation is the process of adding an ext and an ext information. An example of this is at layer 3, where a destination IP address is generally added. Adding extra info like this aids in the delivery and handling the the original data.

Picture a piece of data as a block that ones and also zeroes. During encapsulation, much more data might be added to the prior and/or the back of this block. Data included to the front is dubbed a header, if data included to the finish is dubbed a trailer.

When info is obtained by a host, the procedure is reversed. Headers and also trailers are gotten rid of as the data moves earlier up the great until at some point the original details is left for the application to use.

You can see this process in the chart below:



There room a couple of interesting things we deserve to see here. First, we check out that each layer communicates v the layer above and the great below. They don’t interact with any other layer.

Second, a class on one host will logically communicate with the exact same layer ~ above the 2nd host. The applications layer, for example, communicates with the applications layer on the far host. It no know any kind of of the details the the other layers handle. This can be a little an overwhelming to recognize right now, but it will make sense over time.


Breaking it Down

Key point: Data is damaged into chunks, and passes with the layers 

When an application requirements to accessibility the network, it’s no overly pertained to with how the data is formatted. All it is worried around is even if it is the application on the receiving host have the right to understand it.

But this would not job-related well because that a network. A network demands data come be damaged into controllable chunks. There space a couple of reasons because that this. Imagine the you’re transporting a big amount that data, and also there’s a problem. If several of the data doesn’t do it, you have to start again. However, if the data is damaged into controlled chunks, only the chunk the went negative needs to be resent.

On height of this, remember the there are numerous applications which can want network access. If the network is bound up with one huge transfer, every little thing else will need to wait. But, with manageable chunks that data, applications deserve to take turns accessing the network. Much more important data can even be prioritised over less crucial data.

Generally, each chunk that data is dubbed a Protocol Data Unit (PDU). Each layer often tends to have its own name because that the data it works with:

Layer 1 has bitsLayer 2 works with framesLayer 3 has actually packetsLayer 4 will use Segments or Datagrams, relying on how the data is used

We’ll currently start to obtain a deeper knowledge of what each layer does.




Layer 7 – Application

An interface in between the application and also the network

The Presentation layer, AKA the ‘syntax’ or ‘translation’ layer, is used to transform data between the application and the network. Some examples of as soon as this would be used incorporate encryption, compression, graphics handling, and also character conversion.

In a nutshell, it layouts the data right into something that the remainder of the layers can conveniently use. This makes it feasible for the data come be handled correctly, also if the basic protocols change.

The sending host styles the data therefore it can be construed on the network. The receiving organize converts the data ago to a format that the application have the right to use.


Layer 5 – Session

Here’s whereby the funny begins! We’re external of the upper layers, and into the actual networking! The transfer layer is used to transport data throughout the network without errors.

Part the its job is come segment the traffic into smaller blocks. This is completely transparent come the upper layers. As soon as all the blocks are received at the other end, the recipient carry layer reassembles the blocks right into the initial data. This renders the circulation of traffic show up to it is in a present of data.

The most common protocols used at class 4 room TCP and UDP. They’re huge topics on their own, so we won’t discuss them in depth here. Both TCP and UDP usage port numbers to identify which traffic flow each block the data belongs to. TCP and also UDP both add resource and destination port numbers, in addition to other information, come the data segment in the type of a header.

Using harbor numbers renders it simple to track every flow, which subsequently makes it straightforward to handle different flows in different ways. Because that example, if one flow is overwhelming the recipient, it deserve to ask the sender come ‘back off’ ~ above this flow. Network tools can additionally prioritise one circulation over another, or block a specific flow entirely.

Another task of this layer is multiplexing. Multiplexing method that we deserve to have much more than one flow on the network at once. That’s a major advantage of break up the data into controllable blocks. Each circulation can take turns sending their block of data. This means, for example, that you have the right to receive mail while you are browsing the web.


Layer 3 – Network

The network layer is among two feasible places the we usage addressing. Most commonly, this is wherein we use IP addresses. It’s not surprising then, that this is likewise the layer wherein routing occurs.

At layer-3, a item of details is referred to as a packet. Routing occurs when a packet requirements to relocate from one network to another. The yes, really routing (or website traffic forwarding) in between networks is excellent by a router.

To achieve its purpose, one of the things that the network layer walk is include another header. This header contains the source and destination IP addresses, in addition to other information around the packet. The headers added by class 3 and also 4 room not usually adjusted as the packet moves around the network. There room technologies prefer NAT that room an exception to this, however we won’t obtain into castle here.

As we’ll check out soon, layer-2 have the right to only take care of a block that data up to a best size. So, part of the network layer’s task is come make sure that the packet is not also big. If a packet is as well big, it will certainly be broken into smaller sized chunks, dubbed fragments. They space reassembled when they with the destination host. Each fragment has actually its own collection of headers.

Layer-3 is not forced to be reliable. This means that that does not should implement any error checking prefer the transport layer does. This is because other layers already do error checking.


Layer 2 – Data Link

Data requirements to it is in transferred in between nodes ~ above the exact same network segment. This may be a situation where 2 hosts space on the same segment, or a host is sending out data come a router. This is what the Data attach layer is every about.

But gadgets don’t simply throw a grasp of bits at every other. Castle a little smarter 보다 that. Instead, layer-2 establishes a reasonable link in between the endpoints. This renders it possible to have hosts connected to a switch, quite than directly linked to every other.


Just like layer-3, the data link layer offers addressing. One of the layer-2 address species that you will come across regularly is the MAC address. This originates from the deal with that is stamped onto your network card. As soon as layer-3 passes a packet come layer-2, one more header is added, which includes these addresses. This is no longer called a packet; it is now dubbed a frame.

The network is usually comprised of smaller networks that are joined together. For example, friend may have a main office v a network, a branch office through a network, and also a WAN network connecting them together. This networks may additionally use different link types, such together Ethernet, WiFi, and PPP. That is among the factors that we require the data attach layer.

It will construct a channel of communication throughout one component of the network, for instance from a organize to the router. Climate a different layer-2 connection will be offered to obtain from the head office router to the branch office router, i beg your pardon uses different cabling types. Another connection is then provided to obtain from the router to the destination host. This is quite different to class 3 and 4, which just thinks around the ultimate resource and location hosts.


The data connect layer is broken into 2 sub-layers. The Logical attach Control (LLC) layer is supplied to get data from layer 3 and hand it come the Medium accessibility Control (MAC) sublayer. LLC’s primary job is to translate the class 3 protocol (most commonly IP, however that’s not always the case) right into something the the MAC sublayer can work with.

The MAC layer to add the headers and also trailers come the packet to develop the frame. It will certainly also add information that deserve to be offered for error convey an handling.



Layer 1 – Physical

Defines the physical specifications that the network

The primary job the the physical layer is obvious. It demands to ‘get the data top top the wire’ (or fibre or WiFi as the situation may be). It deals with the raw stream of ones and zeros.

So, what this way is that the physics layer is interested in physics details. These incorporate radio frequencies, pins, voltages, and also how the bits are placed on the physical medium. This means that it requirements to work-related out just how to encode the information. Because that example, which electrical or radio states represent a one or a zero? Is the signal digital or analogue? how do we understand when a structure starts and ends?

There room a few other devices past network cards, cables, and also radio frequencies that fit in below too. These are ‘dumb’ devices which have to not change the data. These encompass hubs, repeaters, and media converters.


TLDR – The Important Bits

These work the OSI version is simply a model. It’s supplied for training and also understanding how parts that the network to the right together. It’s comprised of 7 layers, extending every network role from physical to the application.

When data is sent from or come at a host, it works its method through this layers. Every layer has their own job to do, which may include addressing, error handling, and also reformatting the data.

A big stream that data is broken into controlled blocks that data, which deserve to each be taken on individually. In a process called encapsulation, these blocks have actually extra information included to them, in the form of headers and trailers.

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Each great communicates directly with the layer above and the class below, and logically communicating with the matching layer on the remote host.