 Now that we have talked about the concept of application layer, let us see how it actually works. Hello and welcome to week 3's video on port numbers. So we know port numbers are used to distinguish communication for different applications. For example, the websites use port number 80 so your browser knows that this packet is being sent to a server and it has come from a server and when your browser is sending out the request it attaches its own port number. So now we have seen these things in packet tracer so what we thought it would be a good idea to actually see them on via shark and what we'll do is let's open via shark and let's see how on real time traffic we can distinguish traffic for different applications. Now we'll use the Wi-Fi adapter that is used in this card so we see a lot of traffic that is going around, you have different protocols going around here. So in the meanwhile what we can try is try opening say some website. Let's for example just open our old dummy website which is this. Now since we have generated some traffic we can go back and see what's there on via shark. So what we'll try to do is we'll try to filter out traffic. Now we know the protocol that the website is using is HTTP so let's filter out this and let's try to see what is the port number here. So what we see here is the destination port number is 80 and source port number is 53712. So what it means here is that this packet is intended to go to a web server via the protocol HTTP. So when the this packet comes back then the source the destination port would be this and the source port will be this that is when it comes back comes back from the server. So let's try to see some other HTTP request packets and see if we can see. So these are all requests that are going to the web server. Now you see this one here the source port is 80 and the destination port is 53715. So this means this packet has come from a web server and it has come to the application which was using the port number 53715. So this way port numbers are used to distinguish different communications that belong to different applications and even if it's a same web browser and it has multiple tabs here so all the tabs would use different port numbers. So let's try and see if we can find any other protocol that the laptop is communicating through. So let's try DNS which is domain name service and yes we have some packets from DNS. So again we see here the source port number is 53 and the destination port number is something like this is a response packet and let's try to see here. So we have the source port as 53 and destination here and here again the reply has come. So here what we see is the source port is 53 and destination port is 55987 and here what we see is for one of the replies which was 51807 the reply has come back which is with destination port 53. So this is another example of how different protocols use port numbers and so these are some standard port numbers 53 is a standard port number for DNS requests. Similarly 80 is a standard port number for web browser based HTTP requests and let's see another protocol. So let me show you something called ARP. Now the interesting thing here is ARP is a protocol which actually doesn't work on the application and higher layers it basically just works till the internet layer. So all you see here is the IP addresses and the MAC address. Basically what ARP does is it gets the corresponding MAC address for IP address or vice versa. So we call one as ARP the other as reverse ARP or reverse address resolution protocol. So all the protocols that work on the application layer have these port numbers. So this is a brief demonstration of port numbers. There is one more crucial concept with port numbers called port address translation. For port address translation what we would want you to do is go through the article on port address translation and try to see what it means. Once you have read the article go to the next video and we will talk more about port address translation. Thank you.