From the course: Cisco Certified Support Technician (CCST) Networking Cert Prep

Application layer protocols

From the course: Cisco Certified Support Technician (CCST) Networking Cert Prep

Start my 1-month free trial Buy for my team

Application layer protocols

“

(euphoric music) - [Narrator] In this video, we're going to identify some of the popular protocols that live at layer seven of the OSI model. The application layer, and the application layer protocols can be a bit confusing because we often say, well, this is using UDP port this or TCP port that. And it's tempting to think that these protocols really live at layer four where we have TCP and UDP, but it's important to realize that the protocols we're going to be discussing, they live at layer seven, they just happen to be using an underlying layer four protocol for support. And we're going to be identifying what layer four protocol and specifically what port on that layer four protocol is being used by each application. And to begin to visualize this, let's consider a very basic example. Let's imagine that we're on our client with an IP address of 10.1.1.1, and we want to go to a web server with an IP address of 172.16.1.2. When we send our packet over to the web server, if we were to look at the source IP address, it's us 10.1.1.1, and the destination IP address is the web server 172.16.1.2. And let's imagine that this is a secure web server. In other words, it's using the application of HTTPS. HTTPS is going to be using TCP Port 443. That's called a well-known port for the HTTPS protocol. Now, when the return traffic comes back from the web server, what port is the web server going to use to point back to the client? Well, when the client initiated the session, it picked somewhat of a random number, a very high number, in this case, it was 49,158. And that's going to give a return path for the web server to send traffic back to the client. And if the client has different sessions open, going out to different web servers, it has different tabs in its browser, each session is going to have its own port number that the client selected. And I used the term a well-known port. Those are ports that are in the range of 0 through 1,023. This is where we have well-known services that are running on a device. For example, a web server using HTTP is going to be using the well-known port of Port 80, while HTTPS, the secure version of HTTP, as we have here is going to be using Port 443. Those are all in that well-known port range. And in this video, I'm going to be giving you specific ports that go with specific applications that I want you to memorize. There's another range of ports called the registered ports. These are assigned by the internet assigned numbers authority or the IANA. And these ports might represent a protocol that is playing a role of a service being offered on that server. For example, if you think of voiceover IP, when I'm setting up a phone call from one IP phone to another, I might be using a protocol called SIP, the Session Initiation Protocol. Well, that protocol is probably going to be using Port 5060, 5060. That's going to be in that registered ports range. Now, when the client initiated the connection with the web server we had in this example, it picked a high port. Those are called ephemeral ports. You might also hear them called dynamic or private ports. And the port ranges use can vary a bit depending on your operating system. But in general, we think of ephemeral ports being in the range of 49,152 through 65,535. And it's not enough just to know that HTTPS uses Port 443. We need to know the underlying layer four or transport layer protocol being used. So we say that HTTPS use these TCP Port 443, and there are a few ports and protocols that I would like you to memorize. The first one is used for file transfer. In fact, it's called the file transfer protocol. And it's going to be using a couple of TCP ports, ports 20 and 21. And with FTP, you can connect to a FTP server. And unless you're doing something called anonymous FTP, you'll need to provide username and password credentials to be authenticated and do file transfer. A secure version of FTP, which is going to encrypt the traffic that you're sending back and forth with the FTP server is called SFTP. And it uses TCP Port 22, which by the way is also the port being used by Secure Shell. That's a way to securely set up a terminal session with a device. In fact, Secure FTP is using Secure Shell to protect the contents of those file transfers. Another file transfer protocol is TFTP. That stands for Trivial File Transfer Protocol. And it's going to be using UDP Port 69. Unlike FTP, TFTP does not require authentication credentials, and it's not encrypted. It's a way to quickly upload or download a file. For example, we might have a device boot up and that device needs a configuration file. Well, it could go out and download that from a TFTP server. And a few moments ago, we spoke about HTTP. That's the hypertext transfer protocol. That's the protocol that we'll be using with a traditional web server that does not do security. And it's going to be using TCP Port 80. However, today we much prefer HTTPS, which is a secure version of HTTP. And that's what we saw in our previous example at the beginning of the video where we were using a TCP Port 443. Next up is DHCP. That stands for Dynamic Host Configuration Protocol. This is super useful when we add a device to the network. That device needs an IP address, and while we could go in and configure it ourselves, that's not going to scale very well if we had to configure hundreds or thousands of devices. Instead, DHCP is a way for a device to boot up and dynamically request IP address information from a DHCP server. And when it does that, it's going to be using UDP Port 67. And speaking of IP addresses, we're going to have an IP address assigned to all of our devices on the network. And if I want to go out to the internet and contact a particular web server, that web server is going to have an IP address. However, I'm not very likely to remember the IP address of that web server, but I could remember the name of the web server such as kwtrain.com. But I need a translator to say, given this name, which is called a fully qualified domain name, an FQDN, given this name, can somebody translate this into a corresponding IP address? And that's the job of the DNS server. DNS, that's Domain Name System. And it can take from you a fully qualified domain name and return the corresponding IP address to which your computer can connect to in the background. And interestingly, DNS can use either TCP or UDP Port 53. And although there are many other ports and protocols that we could memorize, the last one I want you to memorize as part of this video is NTP, the Network Time Protocol. This is used by network devices to synchronize their clocks, so everybody agrees on what time it is. And NTP is going to use UDP Port 123. And here's a memory aid for you. When I think of NTP, I think of the old Jackson 5 song, A, B, C, easy as 1, 2, 3. Well, I think NTP easy as 1, 2, 3 because it uses UDP Port 123. And that's a look at a collection of popular application layer protocols along with the underlying transport layer protocol and port numbers that are used by those application layer protocols.

Contents