 Dear students, this module is a continuation of the previous module that how internet works. And in this module, I will discuss with you the two modes of communication. Of course, one of them which is basically used for internet communication which is packet switching. And then of course, I will also talk about different components like servers like peer to peer communication that like routers which are part of this, pardon parcel of all of this. So, let us look at some in more detail. Now, this picture shows what is called as circuit switching. Why it is called circuit switching? The reason being because when these two from this point and this point, there is a communication between say point A and point B, then this circuit is dedicated for this communication. This circuit is set for this communication. This circuit is not shared by any other communication system. It is dedicated for this. Now, in the old days if you have seen any movies, this was done manually. So, you might have seen boards with wires and lady sitting and they are pulling the cables from the holes and putting in different holes. So, what they were doing basically, they were establishing a circuit, dedicating a circuit for a certain communication. Now, that circuit switching is very costly. The reason being that this circuit if between two points on different continents is dedicated to be used by only two people and one of them say for example, goes to get coffee or tea. Now, that circuit is dedicated, nobody else can use that circuit. That is also very expensive and if the people are typing email and they sit and think for a time that what should they type and during that time nothing is happening, that circuit is not available and all other people in the world who would like to use that circuit cannot use that circuit. So, that is a big, big disadvantage of circuit switching that one it is very expensive and number two that it slows down the traffic. So the point I am trying to make is that circuit switching is not made for internet, is not optimized for internet. So, just to give you I have discussed circuit switching just to give you an idea about circuit switching. Now, we move on to the next mode of communication which is packet switching. Now, packet switching what happens is that I have this message over here. I have this message over here and I divide this message into packets M1, M2 and M3. Now, maybe M3 uses this route, M1 uses this route, M2 uses this route and over here M2 is lost. It is possible M2 is lost or maybe they arrive in different order first I get M3 then I get M2 which was lost and then I get M1. Instead of this order the benefit of packet switching is that one communication media or medium is being used by hundreds and thousands of different users it is shared by them. So, the benefit is that it optimizes the bandwidth it is low cost and the transmission control protocol which I will be discussing very shortly that takes care of ordering of those packets, assembling those packets and making the request to recent those packets. So, what happens is that ultimately this message which was sent that is received over here intact perfectly without any issues so that is packet switching. So, internet works on packet switching it is low cost it is efficient utilizes the bandwidth and allows hundreds and thousands and more users to simultaneously share the same communication medium. So, what are the devices which are used for the internet to work servers. Server is a main machine we have the main machine which is taking care of mail which is the mail server we have a main machine which is taking care of the network we have a network server a main machine taking care of database server. So, these are the servers client machines are the machines my palm top my laptop computer through which I connect and retrieve data from the server. I have my processing power but I get the data I get the mail I get the service so I am the client then I have peer-to-peer peer-to-peer means I am having a chat session now during the chat session my machine is sometimes a client sometime it is a server. So, that is a peer-to-peer communication and finally I have the routers so routers what they do is routers they connect the machines say in a school through a router to an ISP and so on in an organization connect different computers to router to the main server and so on. So, let us let me show you in a picture how all these things are connected together. So, you can see over here that this PC is the client over here this is the client over here this is the DNS server which I will be talking domain name server domain name service this over here is the internet this over here is one router this over here is another router then another router you can see that there are many routers and over here resides my domain which I had purchased on this web server I will talk more about the details about this. In this environment how these things work these things work using the IP internet protocol they work using the TCP IP transmission control protocol and the domain name service. So, what is IP? IP is basically it assigns the addresses to different devices IP address which I have briefly mentioned which consist of numbers between and dots between them. And then I have the transmission control protocol which assembles and dissembles and takes care of the lost packets and everything. So, that is the TCP control protocol and then I have the domain name system domain name system is required because the IP address is almost impossible for people to remember. So, they have associated text with it. So, when I host my domain at a certain service provider they associate an address to the device where it is hosted. So, that is the IP address and the DNS the domain name system translates the text into the IP address. So, this is how things are tied together this is how they work I hope you understand you have questions you are welcome to ask thank you very much.