Port Numbers Explained | Cisco CCNA 200-301

Hey what's up guys welcome to CertBros. In this video, we're going to be looking at port numbers. So back to our trip through the tcp/ip model and remember we're going through these layers as if we were sending data so top to bottom. In the last video we looked at the layer 4 protocols TCP and UDP. Layer 4 is also responsible for choosing port numbers and that's what we're going to look at in a bit more detail. So why do we need port numbers? Well let's say you live in this house, you will probably want to receive post here So what do you do? You install a letterbox. Now when the postman comes your letters can be put through that letterbox. The letterbox gives the Postal Service access to your house and computers work in the same way. The post in this case is the application data and the letterbox is the port number for that application. It gives access to your computer for that service. Now instead of a house it might be a server and this server could be running a mail, web or any other service we want to send data to. Let's say we want to access the web server. We type in the web address or URL of the site we want to visit and the first thing the computer will do is convert this URL into an IP address This is done by using DNS but we will cover that in another video For now just know it converts that web address into an IP address. The computer then sends the request to the webserver. But this server might not just be hosting a web site using HTTP, it may also, be a mail server using SMTP or even a file server using FTP. So how does the server know which application to send the request to? Well these applications have something called a well known port number assigned to them. HTTP is assigned port number 80 SMTP is assigned port number 25 and FTP is assigned port number 20 and 21 So because the port numbers are standard numbers all computers will know about them. When we made our web request our computer knew we were trying to access a HTTP site, so it added the destination port number of 80 to the TCP header The computer will also choose a randomly generated source port number to receive a reply. It's sent to the web servers IP address and the well-known port for that service. The IP address and port number are often written like this. It's the IP address a colon then the destination port number. The server will receive this request look at the destination port number realize the request is for the web application and pass it to that application. The server would then respond. This time the port numbers are reversed the destination port is now the randomly generated one and the source port is our well-known port number 80 Again when we receive this response our computer looks at the port number. The port number let's our computer know which application to send it to and in this case our browser and even which tab of that browser to display it on. So the IP address gets the data to the computer but it's the port number that gets the data to the right application. Now this is a simplified version of how this works just to give you an idea about port numbers what they're used for and how they work. Ok so let's look at well-known port numbers in some more detail. There are several port numbers that are called well-known ports. These are common protocols that have been assigned port numbers. Here is a very small list of some of the most common ones you'll find we have things like HTTP port 80, HTTP port 443, SMTP port 25 and the list goes on and on. It's important to learn these ports not only for the exam but also for your everyday troubleshooting. You may be looking at a packet capture and you'll need to know what type of traffic you're looking at. if you see communication with an IP address on port 53 you'll know is DNS traffic. Now this is by no means a complete list. No, in fact port numbers 0 through 1023 are well-known port numbers. Thankfully we only really need to know the more common ones. There are also ports ranging from 1,024 to 49,151 these are registered ports that companies have registered. And finally we have ports 49,152 through 65535 which are dynamically assigned ports these are used to randomly generate unique port numbers for that local computer that it can use as its source port. So to give you something to try at home let's see these ports in action. If you were to go and open a web browser and visit any website you want for example certbros.com and then open up powershell on your windows computer If you then type netstat to show all the current connections. And -n to show the numeric values. You would then see your computer's connections. You will see the protocol being used, the local dress which means your computer's address. Note that it shows a colon and a port number in this case it's our source port. It also shows the foreign address which is the IP address of the device you're connecting to. This also has the port number here it's our destination and well-known port number of 80. Now most websites you see today will be using HTTPS rather than HTTP which uses port number 443. HTTPS means the traffic will be encrypted between you and the web server. Because at the time of making this video the site was still in maintenance we're still using HTTP, however by the time you see this it will likely be using HTTPS on 443 We can also manually change the port we connect to. For example if I type certbros.com:80 it takes us to our maintenance page But if instead I wanted to go to certbros.com:53 which is the port number for DNS the site can't be reached because the server is not sending the request to that web application. In fact is trying to send the request to a DNS service that isn't running on this web server So that's it for port numbers go and take a look at your own connections and port numbers being used. This video is part of our full CCNA course which can be found in the description so please feel free to go and check that out. If you like this video don't forget to Like comment and subscribe. The feedback from you guys really does help the channel grow. Other than that, thank you for watching

Channel: CertBros

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Keywords: port numbers explained, computer ports, port 80, port 443, computer networking ports, ports explained, computer ports explained, port numbers, well known ports, ccna 200-301, ccna training, cisco, cisco ccna, cisco networking, computer networking, computer networking course, networking, networking tutorial, network+

Id: RDotMcs0Erg

Length: 7min 55sec (475 seconds)

Published: Tue May 19 2020