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Now in the definition of what a network is we're told that a computer network is used for sharing resources
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between nodes which are computing devices.
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In other words we have devices that are sharing resources between each other.
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And there are two main devices that you need to be aware of initially and that's a server and a client.
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Now what is a server.
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So let's ask of the robots once again.
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Alexa what is a computer server in computing.
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A server is a computer program or a device that provides functionality for other programs or devices
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called clients.
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This architecture is called the client server model and a single overall computation is distributed
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across multiple processes or devices so notice a server is providing a service or functionality to clients
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in what's called a client server model.
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So servers provide functionality to clients and that begs the question what is a client.
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Alexa what is a client.
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A client is a piece of computer hardware or software that accesses a service made available by a server.
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The server is often on another computer system in which case the client accesses the service by way
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of a network.
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Now listen to that that's very important to remember.
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A client accesses a service provided by a server.
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Typically the server is a different computer or different computing device to the client across a network.
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So we are going to access a service hosted by a server across a network.
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Here's an example of a server big server right here inside.
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Here are a lot of drive base.
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Yes a hard drive.
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He has another one and another one and another one.
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And I could continue
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the whole idea of a server is to provide resources or functionality to clients so the server is big
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because we want a lot of storage.
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We want a lot of computing power and a lot of memory in a server especially if we want to scale it up.
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If you've got thousands and thousands of queries from many many clients to a server you wanted to have
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the capacity so memory CPSU storage to handle those queries.
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So hence the big device.
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It needs to have the capacity to service the requests.
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So if only one or two people are requesting a web page from the server that's not a problem.
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But if we've got thousands and thousands of requests one server may not even be enough.
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And then we have distributed services.
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So we have a service such as a web page that's distributed across multiple servers.
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Google as an example will have thousands upon thousands upon thousands of servers to service the requests
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from millions of clients so they will have many many servers to service or provide a service to the
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clients.
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When you go to the Google Web site and you type in a request that's querying databases and retrieving
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information from databases to give you a answer about some query that you've made.
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So the server is providing a service to a client.
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Now you don't need dedicated hardware like this to have a server a client device such as a laptop can
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act as a server.
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So the role will determine how that device is acting.
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A physical laptop such as this laptop could be a client requesting a service from a server but it could
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also be hosting a service so it may be running some kind of server and then sharing a file as an example
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with a another P.C. so it's hosting a file sharing service.
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So clients access servers to make use of a service that the server is providing a website will serve
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a web page to a client which is then displayed on your computer as an example later in the course we're
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going to learn about network automation and network program ability.
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That's becoming really important and in that kind of scenario you'll have one application or program
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providing a service to another program typically using what's called an API application programming
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interface.
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We're going to ignore that example for the moment.
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We'll get to that later where we've got machine talking to machine basically application talking to
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application 1 application interacting with another application.
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Let's for the moment concentrate on physical devices.
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Now a server such as the serve over here one physical computer will be listening on different port numbers
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for different protocols.
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What is a protocol.
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It's basically a set of rules used for communication between devices.
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As an example I'm speaking English here.
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Hopefully you can hear what I'm saying and hopefully you can understand what I'm saying because I'm
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using a set of conventions in my speech.
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So in English we speak a certain way.
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As an example some languages will use numbers differently to English.
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The protocol that we are speaking here is English so I will say 21 but in a different language like
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Afrikaans I'll say yes on Twitter which is basically saying one two to denote 21.
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So the way that words are pronounced or the way that numbers are pronounced is reversed in a different
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language to English.
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The point is you and I are using a specific protocol we are listening to English as the transmission
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and you can understand what I'm saying.
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But if I switched my language or my protocol and started saying We are who cannot fund off you might
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struggle to understand me unless you understand South African.
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Or should I say Afrikaans.
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The point is is that a server doesn't just automatically switch like we do in our brains from one language
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to another.
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It has to listen on specific port numbers.
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Your ears are listening on the same frequencies for both English and offered content then your brain
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just switches from one language to another.
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A server is not as clever as that.
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It has to listen on specific port numbers for specific protocols.
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As an analogy here's an analogue radio.
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Old technology I know but notice here I'm not receiving anything because I'm not listening on a good
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port.
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No but as I tune to specific port number
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I can pick up a transmission I have to be careful because of copyright what I transmit.
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Yeah but that is a different radio station or a different protocol.
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To say that the radio station.
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So different radio stations transmit on different frequencies.
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Think of that as an analogy to what a computer server is doing a computer server like this is listening
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on specific port numbers.
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If you want to get a web page your browser is automatically configured or programmed to talk to port
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80 or to port for 4 3.
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If you're using an application like FCP the application is automatically going to talk to the server
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on Port 21 and the server is listening on Port 21.
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So a server can run multiple services and provide multiple services to clients client server model client
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requests something from a server the server provides the file or the service to the client.
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