Training Cisco 200-125 CCNA v3.0. Cisco Certified Network Specialist (CCNA). Day 1. Network Basics
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This 46-day video course is updated as of 2018 and contains 49 video lessons lasting from 17 to 65 minutes.

Welcome, I am your teacher Imran Rafai, today we will begin our series of lectures with the topic "Network Basics". This video course is ideal not only for those who are going to take exams for a Cisco certified network specialist, but also for those who are interested in networking or want to start a career in networking. A CCNA certificate is a very valuable thing, and I urge all those who strive for excellence in this field to receive this certificate because it is of great importance for assessing your professionalism. Before proceeding, I will ask you to focus on the first 3 days of training, because these days I will cover the fundamental concepts that will become the basis of your network career and will be useful to you, maybe even after 20 years.

Feel free to ask me questions, write directly to me through the contact form on the website or go to my social media pages: imran.rafai

So, let's start with the simplest question - what is a network. When I ask this question to my students, I often get answers like: “The network is Facebook, Twitter, Picasa, and so on,” that is, everything you see on the icons on this slide.

But a social network is not the network that we are going to learn about, and not the one that I will talk about. What interests us is the network between computers, the basis for applications such as Twitter and Facebook, because all networks are built on the same principle.

We will study just such a network, which is shown on the slide, and if at the end of this lesson we have time, we will talk a little about the applications used here. When I created this slide, I thought about how, without using a lot of technical terms, I could explain the concept of the network to students who are only going to study the network industry.
I can’t tell you this way: “this thing is connected with this thing or this thing”. I thought that I needed a better way to explain the concept of the network and remembered the story that my teacher told me when I studied the network. It was about how in the old days the kings exchanged messages. They handed the message to the messenger, who rode on horseback along the roads and paths, then drove through the city gates, entered the chambers of another king, and handed him this message. If you draw an analogy with a computer network, the message is the data that can be anything: an MS Word .doc or .excel file, an image, or a video. The roads that the messenger traveled are computer networks. Of course, a computer network is much more interesting than a simple paved road, but it can be concluded

In this picture you can see Windows, Linux and iMac computers, a laptop, a mobile phone, a tablet, a pocket PC. At the top right you see a web server, a database server, a file server. Despite the fact that all these devices are running different operating systems, they are absolutely no problem communicating with each other. This is the magic of a computer network that works on a global standard called “OSI Network Model”. This standard clearly states that anyone who manufactures computer devices must ensure that they interact with each other in a single language of network communication.
Let's take a closer look at how the Internet works. In this garden you see the main network model, these are 2 computers connected by cable.

In this case, we have a Cat 5 network cable, you can see how it looks in section. These cables come in a variety of colors — blue, red, or some other. Under the protective sheath are 9 thin wires that are connected to the connector type RJ45. On the back of your computer, if we are talking about the PC system unit, there is a jack for connecting this cable, which is located on the network card. Thus, two computers can communicate with each other, and such a core network is called an Ethernet network. I will try to write this word on the screen with a mouse, I really succeed in it, maybe by the next video I will buy a stylus to write in a more legible handwriting. So, by connecting 2 computers with this cable, you will get a very simple network, but what if you have more than two computers, say 5? Probably,

In this case, all computers communicate with each other through a network device - a switch, or a hub. The switch and hub are completely different devices, they have different functions, we will look at them in the following video tutorials. For now, it’s enough for you to know that through such a network device, in this case these switches, computers can communicate with each other.

The guy on the left has a document that he wants to send over the network to the guy on the right. All he has to do is send the document through his IP address. You ask what is an IP address? An IP address is what makes computers recognize each other on a network. We will talk about these addresses in the following video tutorials, for now just remember that they are computer identifiers.

Another important reason people use computer networks is to use devices like this printer. Suppose that this is a very expensive printer or scanner that the company purchased, and if we do not have a common network, it can only be connected directly to one computer. Suppose that this user prints no more than 1 page per day, and all other users in this company print up to 2 pages per day. Then the company will be forced to purchase personal printers for each user, and their use will be minimal. Therefore, the company invests money in the network infrastructure, which allows you to have only 1 expensive and high-quality printer for all employees connected to the network, and thus saves money and maximize the budget,

That is the main requirement for computer networks and the main reason for their use.

So, a computer network in an office, or even in one room, but usually this is a network in one company, called a LAN — a local area network. This is such a network, which is located in one geographical area. Suppose there are two offices in your company - in New York and Boston.

To connect them, you create a network between the two offices of your company. Of course, if you have a lot of money and you have received permission from the authorities, you can independently connect these offices by cable, laying it underground from New York to Boston. But most companies have neither such money nor the desire to spend it this way, so the best way to do this is to connect both offices to a local ISP provider, or an Internet provider. ISPs already have sufficiently powerful networks with high bandwidth between them, so your data from the office in New York goes over the network to the local ISP, then it is sent to the Boston provider and from it goes to your Boston office. Therefore, the computer in the New York office is absolutely free to connect to the computer in the Boston office using an ISP.

Thus, a wide network is being created, by the word “wide” I mean a large geographical area connecting many local area networks. It can spread throughout the country, it can be organized in different cities of the same country, or in different countries. In fact, all this is LAN.

How does this work, how do you connect your computer to your ISP? Usually you have a small outlet on the wall to which the cable from your network card is connected, and on the other hand, a switch of your company is connected to it. An underground cable is routed from this outlet to the ISP, and the same underground cable is laid between the ISP offices in New York and Boston. In Boston, a similar pattern is repeated - an underground Internet cable from an ISP office enters the company's office and connects to an outlet where a router is connected to which office computers are connected.

This is how the LAN works. The Internet also uses ISP, because the Internet is nothing more than a very extensive LAN network that covers the entire world, in which there are many publicly available resources. Google says that the Internet is a global computer network that provides a variety of information and communication tools, which consists of interconnected networks that use standardized communication protocols.

Here is a long definition. Thus, the Internet is an association of multiple local networks. You have a LAN here, here and here, and they are all connected to each other. American networks are connected to European, African, networks in Asia, India, they are all connected to each other - this is what the Internet represents. If someone from India wants to connect with New York, the data will follow this path, and if there is a link between Europe and the US, the data will go along a longer path through Africa and South America. So, the Internet is a set of mutually connected local networks.
Let's talk about the applications I already mentioned. There are many Internet applications, and we all use such applications as Skype, eBay, CNN, YouTube.

This figure shows a very small proportion of Internet applications, you know a much larger number. Consider how, for example, YouTube, which is owned by Google, works.

Google somewhere on the Internet has organized a public, accessible to all server, so if I want to watch a video on YouTube, then I go to my web browser and type . The process that takes place in the background is much more complicated, but we will try to simplify it and show how it works in the following video tutorials.

Say that when someone types and presses “Enter”, the computer sends the HTTP request to the public YouTube server. When the server receives this request, it thinks: “great, this is an HTTP request, so I’m sending back the HTML file”. He creates an HTML file and sends it back to me. My browser accepts this file and says: "I know what this HTML file is, and I know how to process it to show the video from the YouTube site." In the same way this happens with eBay, CNN, Skype. This is how the Internet basically works: there is a public server on which there are files that your computer requests, and these files are sent to you in response to a request. This is how the Internet helps to globalize the information world.

Before discussing the Internet, consider its most important feature - speed. Let's discuss what the difference between a bit and a byte is. A bit is the minimum piece of information that a computer understands; it can be 0 or 1. A byte is a unit of information consisting of 8 bits.

Thus, one byte may look, for example, as follows: 1 1 0 1 1 1 0 1. People are often confused in these units, so remember: data is always measured in bytes. When I say that I have 1 GB of RAM, it means 1 gigabyte of RAM, but when I talk about speed, that I have 10 Mbit / s, it means that I transfer 10 megabytes per second. Thus, data is measured in bytes, and the data transfer rate in bits per second. If you confuse these two concepts, you will have a hard time. If I say "byte per second," it means "8 bits per second." So remember that the speed is always indicated by a small letter b - this is a bit, and the size - a large letter B, this is a byte. One kilobit = 1024 bits, just like 1 kilobyte = 1024 bytes.

Consider an example. How long will a 128 KB file be transferred to the network at a speed of 1 MB / s? We convert the file size from bytes to bits: 128 x 8 kilobits = 1024 kilobits, or 1 megabit. Thus, at a given connection speed, the transfer time for a file of this size will take no more than 1 second.

If you understand the difference between bits and bytes, then it will be very easy for you to understand the content of the other video tutorials. If we are talking about speed, we need to mention three more important factors that slow down speed. All these factors are always interrelated. When you connect the cable from the router to your computer, it reports that it is connected to the network at a speed of 100 MB / s. Today it is the maximum speed that a LAN network card can provide. But do not forget that the speed we are talking about is the speed between your computer and the router. Your internet connection can have a speed of 1 Mbps, so if you try to connect to the Internet at a speed of 100 megabits, it will not work for you, because the Internet can transfer only 1 megabit. Suppose you want to transfer a file from your office in Bangalore to an office in New York. So your file through the gateway - the router arrives at a speed of 1 megabit / s to the ISP in Bangalore. The provider in Bangalra is connected to a provider in Mumbai by a high-speed line of communication capable of providing a speed of 100 Mb / s.

The speed between ISP in Mumbai and ISP in Dubai is slower because there is a line between them that provides only 10 Mbit / s, but then there is again a 100 Mbit / s high-speed connection between Dubai and Cairo. Further, between Cairo and Madrid, there can also be high-speed Internet, then the speed drops and so on. That's what happens on the Internet, which is a bunch of routers with different data transfer speeds. Even if I have a 100 Mbps high-speed connection in Bangalore, the transfer speed between the Bangalore and New York offices will be determined by the minimum connection speed on the link, in this case the speed between the IPS in New York and the computer in New York office, that is 1 Mbps.

This is how internet speed works. Of course, in a real connection between Bangalore and New York does not occur through Mumbai, Dubai and Cairo, I cited these cities as an example, and the actual way of transferring data may be different. But in general, data packets are moved in this way from one place to another. Thus, the speed depends on many factors, including the speed on different parts of the network route. So, the second critical factor is delays. Let's assume that we have another office in Boston, I will locate it to the right of the office in New York. When sending data from New York to Boston, they go through fewer nodes and, therefore, it will reach the goal much faster. Hence the conclusion - if the distance between the two devices is large, you may experience communication delays.

Therefore, if you come from Singapore to or, the network will direct you not to the Google server in the USA, but to the server closest to you, maybe to the IPS server in Singapore itself, while the delay will be insignificant. But if the link between Cairo and Dubai is broken, there will be a longer detour from New York to Bangalore, possibly through Russia, China and India. This is a very long process, and a third factor arises here - availability, that is, the availability of the necessary resources for the Internet.

Availability means that all communications between servers must be operational.
One more thing that we will discuss in this video lesson is the network topology. There are three fundamental types of topology: "star" (star), "ring" (ring) and "bus" (bus).

"Star" is one of the oldest computer connection schemes, which exists to this day. Here all computers or devices are connected to the central switch. If one of the computers loses connection with the switch, it will not affect the connection of other computers, but this computer will completely lose connection with the network.

In the “ring”, each computer is connected to another, and if the connection between two neighboring computers is interrupted, they will still be able to communicate with each other through the next computer.

On the “bus,” each computer is also individually connected to the network using its own small network device. Thus, if one section of the network fails, the computers located nearby will be able to communicate with each other, but will lose contact with the computers of the network segment that is located behind the damaged section.

Let's see how this is implemented in the modern world, returning to the first slide with the image of the network. As you can see, the switch and the three computers closest to it are connected to the general network of the star topology, and the servers located in the upper right corner are also connected to the switch. Practically in the same way, mobile devices are connected to a WiFi AP access point. The two central switches are connected according to the “bus” scheme, so if the connection between them is interrupted, the devices on the right can interact with each other, but cannot communicate with the left network segment. Sometimes the switches are connected by additional lines of communication, so even if the connection in one area is interrupted, the connection can be carried out on other network segments.

Thus, in the real world, a hybrid or mixed network topology is used, which uses both the "star" and the "ring" and the "bus".
This is all the information I wanted to present to you in our first video lesson. If you do not understand something, you can always review this video. Feel free to contact me at the contacts listed above, you can also subscribe to our YouTube channel to view our latest videos. You can also visit our website. That's all, we'll meet on the second video lesson.

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