CompTIA Linux+ XK0-005 – Unit 09 – Networking Part 1

1. Networking

All right, in this section we’re going to talk about networking. Now the goal here is to give you a good overview of networking. Remember, this is something that allows all different devices to be able to communicate to each other regardless of platform linux, Unix, Windows, Mac, regardless of type of device, router switch, some type of server that’s out there, storage area, networks. The goal is that we can all communicate. And so we’re going to give you an overview of the basics of networking so you know how to make your Linux machine be able to be configured to make that network connection. We’re also going to talk about some of the things you need to configure to be able to help work with and facilitate your ability to communicate like name resolution, IP addresses and all of those other options.

Now there are going to be times when you might at least I hope anyway, that you might be saying, well, Ken, I wish you would go further into this subject. Well, there’s an entire networking plus class that will give you all of the detailed information. I’m going to give you an overview hoping that you don’t feel like I’ve left something important out. I won’t leave anything important out when it comes to how to make your Linux machine connect to the network. But if you want more information, I’m going to encourage you of course, to get that information for any type of networking concerns if you want to have a better and better understanding of how things really work.

2. Topic A: Networking Basics

So we’re going to start off with the network basics. The goal here is to create a foundation for the understanding of what’s really happening behind the scenes. When your Linux machine is going out to the Internet, going out to another server, or whatever it is that you’re doing that involves communications beyond the actual server or workstation that you’re sitting at.

3. Networks

When we talk about networks, you’re going to hear a lot of different types of terms and this is still not necessarily as universal as I would like it to be. But let’s get into some of the terms that you’re going to hear, like nodes. Now, nodes are supposed to be endpoints, endpoints of IP communications. By an endpoint, what I mean is that communications will go to that node or come from the node, but it will not relay the information. Another group, the ISO, would call it an end system. So again, right there, IP, endpoint, endpoint node, end system, lots of terms to mean the same thing. It is where communication begins and ends and that’s normal for PCs, for servers and everything else that is not the same.

To have an intermediate system, an intermediate system would be like a switch or a router where it is not necessarily the target of communication, but the device that helps relay or further the transmission from one place, one node to another. Now, in order for a node or any device to connect to the network, you have to have a network interface card. The network interface is just something that is able to communicate on whatever medium you’re using. Wow, medium. Well, that’s great. It could be back in the old days when we used coax cable, you would have a network interface that had the ability to connect a coax cable. Today, you see in the copper wire range anyway, these things we call category five or cat seven cable.

Most of it is cat five e and it is a copper wire. Eight wires broken down into four twisted pairs that help connect you to the different types of devices switches, hubs or whatever. You also will see fiber, which is basically our ability to transfer information through photons instead of electrons. Some of you may not see any type of connectivity because it’s using radio frequency or what we might call infrared or some other transmission that is invisible to us. But it’s still a valid medium that you have. So the network interface is what you use to connect to whatever medium you’re using. Now, the transmission on that medium or the transmission medium is what I described.

Copper wires, radio frequencies, infrared, photons, everything but smoke signals. Now, maybe some of you got that joke right because you have a little puffy cloud for a one, no cloud for a zero. Okay? Anyway, transmission mediums are ways of being able to connect everybody and there is no this is the best, this is what you should use. It makes sense based on price, speed and your needs. And of course, there’s a bunch of software, the software that we have to be able to use to be able to make the communication happens. We often call these protocol suites that we use to help us in making those connections. And we’ll get a chance to talk about the most commonly used one, which is the TCP IP Suite.

4. Protocols

Now, part of these communications deal with languages that everybody speaks in common. We call them protocols. Protocols are the language of the network. Now, they also represent either proprietary solutions or open standards. Now, proprietary solutions for protocols. As an example, in the old days, Novell had a method of communicating data and addresses by using a proprietary thing. They called SPX IPX. Today we use IP and TCP or UDP as our standards for our communications because they were open standards, meaning nobody owned them and I didn’t have to pay anybody a royalty to use them. Now, these ones that I just spit out TCP, UDP, IP, they all have different functions, but they are standards. They are protocols of how we communicate. We also have protocols or encapsulation methods that deal with the type of medium that we’re communicating on.

You’ll hear words like ethernet or frame relay or ATM or sonnet. You’ll hear these different types of things and we talk about the type of connections and the protocols that we use. All of these are actually put together in what we call a layered architecture. The layering that we’ll see, we’re going to talk about this thing called the OSI model. But the layering is a way of being able to get one protocol to communicate with another protocol going up or down through the layers. By following some of these standards or this layered architecture, we’re guaranteeing that these different types of systems are able to communicate to each other because they communicate through the same layered architecture in the same manner. That way my Windows machine and my Linux machine can talk to each other without any problems.

5. OSI Reference Model Part1

All right, so the foundation then, for this layered architecture is what’s called the OSI reference model. Now, the OSI stands for Open System Interconnect. And what it is is basically a way of describing how communications will occur from node to node. Now, there are seven layers. We divide them in what we call upper layers and lower layers. In fact, this particular section is focusing on the lower layers. But let me just tell you what they are from the top, the upper layers all the way down.

6. OSI Reference Model Part2

We actually would call the top layer seven, and that’s the application layer. Generally speaking, you working with the operating system through some program. The next layer down, layer six, is the presentation layer. If you’re working with some sort of word processing program, you’re going to produce a document that is stored on the hard drive. And that document was designed to be compatible with that application. Some of you might call that the formatting, but that’s what we define at the presentation layer. MP3 S is a type of presentation layer. It talks about the type of file and what it generally is going to do. A music file.

7. OSI Reference Model Part3

Beyond that. The next layer down, layer five, is the session layer. The session layer is the ability of your operating system to keep track of all of the users and all of the applications, so that as communications go back and forth, they don’t get switched or lost. In other words, if you have two or three browser pages open, one going to a new site, one ordering a book, you don’t want somehow that information to switch and have your new site go to the book order or whatever. That’s the sessions that we keep open and keep separate. So that’s the upper layer, that’s the rest of the upper layers. So seven, six and five are the layers.

8. OSI Reference Model Part4

Now the lower layer, starting at layer four is the transport layer. Now the transport layer is very important. This is where we actually choose the language that the two nodes are going to have an exchange of information with. Commonly we talk about TCP and UDP. Now without getting too deep into those, the transmission Control Protocol. TCP is often used for pointtopoint node to node code communications. It deals with issues of taking large data blocks and dividing them into small chunks of data so that we can actually transmit them over the media. That’s usually a copper wire Ethernet. Those small blocks have to be reassembled on the other side.

So along with each block we give it a sequence number so that you know the order in which they should be reassembled. TCP deals with that type of thing. We often call that a segmentation process. UDP, the User Datagram protocol, was designed for one to many or one to all communications. That being a multicast or a broadcast. The biggest difference between them is that TCP is waiting for an acknowledgment so that I know whatever I sent to you was received. UDP does not want acknowledgements. It’s what they call a connectionless protocol. I don’t like saying that because it sounds like it doesn’t do anything, but it is appropriate, it’s best effort.

I sent the information. If you got it, great. If you didn’t, too bad. I won’t know, I’m just going to keep on sending it anyway. Those are some examples of the transport. The most common ones that we’re going to be using. Going down below that, layer three is the network layer. The network layer was designed to help us logically assign addresses to the network that you belong to. Now you’ll hear terms like broadcast domain subnets. That’s great. It’s a logical area that contains what we call the local area network or a particular group of people that all share some common communication needs. Whether you want to call it the land subnet broadcast domain, that’s great.

But we usually give it a logical address and that is in the range of what we call an IP address, an Internet protocol. Now, the IP address does two things. It not only identifies the network that you’re on, but in that network it identifies uniquely what your host is. If you ever think about sending a letter through the post office, the idea is that part of it is the zip code, what part of the country are you in? And then specifically what’s your address in that particular part of the zip code. So that’s kind of the idea of the IP address. Now technically the IP address gets you close, it gets you to the network, and then it’s followed by layer two, which is the data link layer.

9. OSI Reference Model Part5

The datalink layer is where we actually look at your physical burnedin address and know exactly who you are to send things to you. Remember, I said at the network layer IP was logical? Your IP addresses can change. That’s fine. But your physical address, what we see at the data link network should not change. I mean, it can, but it’s generally burned into your network access card, the in this case, the network interface card for Ethernet. And we call it your Mac address. And so that helps us physically know exactly who you are. And it helps us in the areas of switches in knowing exactly where to deliver the communications.

10. OSI Reference Model Part6

Anyway. So all of these are facilities that are open standards that were designed to help deliver information to and from a machine. Obviously, the application is going to generate the need for wanting to transfer information or receive information. The presentation is going to deal with the type of package it is. The session is going to make sure we don’t screw it up and get it lost in the different programs. Running the transport layer helps us in actually making the community communications segmenting the package, knowing how to reassemble it, acknowledging that we receive it. The network layer address helps the devices in between.

You and I like routers get the thing delivered to us because you could be across the country, you could be across the world when it gets to us, our local devices, our local switches, need to know physically what your address is. That’s the data link layer. And then finally it’s going to be turned into a bunch of what and zeros and transmitted through radio frequency, through electrons, through photons, through infrared, whatever is being sent in. And that’s the last of those layers. Layer one, the physical layer. So that’s what the OSI reference model does is it creates a layered architecture of an idea of how communications should work.

11. OSI Reference Model Part7

Now, I kind of talked about sending the data. The opposite is true of receiving it. You look at those ones and zeros. You look to see if it matches your physical address. If it does, you make sure it matches your IP address. If it does, then you can figure out how to put it back together and eventually get the thing put together and put into your application. It goes in the reverse order of the person sending it anyway. That model encourages these standards and these communications and allows dissimilar odd systems to be able to have a common method of communications.

12. Internet Protocol Suite

Now, in specific, I said that we would look at one of the software suites called the TCP IP Suite. Some people just call it the Internet Protocol Suite. It contains something very similar to the OSI model. In fact, it predates the OSI model and having had its own set of layers. Now, its set of layers were not as complex as the OSI model. It had fewer layers, but it still had the idea of applications, transport, Internetwork and its own data link layer, which included physical stuff as well. But what we really want to know is what kind of protocols do we care about? So at the transport layer, as I said, we had TCP and UDP. I think I did a good enough job at describing the basic functions.

TCP good for a host to host communications, where we have Acknowledgments and UDP for a one to many type of communication. Multicast broadcast IP was designed for the logical addressing of your local area network. You most often will call it your network address. Some of you will call it your subnet. More generically, it’s your broadcast domain. But it helps your routers know how to get from where you are to where you want to go by following its best path. They do that through a whole series of routing protocols we’re not even going to get into. Now, at the application portion of this IP Suite, we have programs that help us be able to, as people, work with network devices. One of them is called the DNS service.

The domain name service. It is far easier for you and me to remember that I want to go to www. yoursite. com than it is to remember your exact IP address. And so by being able to say, I want to go to this website and putting in the name, you call it the URL, the DNS services job is to translate that to the actual IP address so you can make the communication. Other protocols that you have, like the Network File System, allow us to be able to share resources from one system to the other so you can make a remote connection. And we have a protocol for transferring files for those remote network connections. If there’s any communications problems, any errors in this process, we often want to communicate those errors.

We have ICMP to be able to do that, the Internet Control Messaging Protocol, to be able to send information that says Host not found, or this isn’t open, or if anything, to confirm connections through what we call pinging, which is simply an echo request and an echo reply done at ICMP. Likewise, I said we needed to have a method of being able to figure out your hardware address, especially in Ethernet. So even though DNS tells me your IP address, if you’re local to me and I have to go through my switch, I need your Mac address. So we have protocols like ARP, the Address Resolution protocol that can help, basically help you ask somebody on the network, hey, who’s got this IP address? And whoever owns it is going to respond and say, oh, that’s mine.

Here’s my physical address. What we call the Mac. The Media Access Control. Now, some machines, when they first start up, they don’t have an IP address, but they have a physical address. And so they need to have a protocol that says, hey, this is my Mac address. Can somebody tell me what IP address I’m supposed to use? And that’s called the reverse ARP, or the ARP, if you want. And today, of course, we look at services that answer those questions. We call them DHCP services or Dynamic host configuration protocol. But that’s what it does. You ask for an address, it gives you one associated specifically to your Mac address.

13. Additional Protocols

Now, there are many other protocols that are actually defined in these suites. And I don’t want to get into all of the details about all of them because that’s a networking class. But just so you understand, because you’re going to recognize some of these, like a remote console connection or remote access, where you are going to use a command line, access to a device you would do through telnet. For web services, we had Http, the hypertext transfer protocol, which was nothing more than a protocol about how to exchange actual information, which is nothing more than textual information, but formatted in a reference called HTML. And if we were worried about people stealing our information, there was a secure method of opening this up to Https.

Or we could encrypt the web page we want to send and send you the encrypted page to Shttp. Then there’s a whole bunch of ways of being able to transfer data files. That means I have a server that is offering files for anybody who has the authentication to connect, and that’s FTP, the file transfer protocol. We also have a secure method of doing that as the SFTP. Then we also have a lightweight program that’s not near as robust as FTP, but is great for short term. I need a server that’s got a file, I need to make a very quick connection without any authentication, and that was TFTP or the trivial file transfer protocol. We also have some great tools for management, for what I like to call centralized management.

Now, network management systems, NMS, are very popular, and what I do is I have all of the servers report to the central location, lets me know when they’re up, when they’re down, if something’s failed, if whatever. I want to keep track of that communication is done through SNMP, which is the simple network management protocol. Now, some people like to joke and call it security. Not my problem because with the exception of version three, it’s all done in clear text. Anyway, there’s my little security part that I love to throw in there email, lots of protocols for sending email. SMTP, the simple message transfer protocol for retrieving email off of the server, the Pop, the post office protocol, version three.

Or if you want to connect to the server, leaving the mail on the server is the IMAP version four. For secure communications, as I talked about before, SSH would be the secure version of telnet. SCP is any other secure version of FTP secure copy. And if I want everybody to have basically the same knowledge of what date and time it is and not have everybody running their own little clocks, we have a time service that we use, the Network Time Protocol or NTP. And this is just again, a handful of protocols that you can use through this TCP IP suite that fall in line with even the OSI model. The ones I just mentioned are the ones you’re going to comment, be seeing, and probably have to configure on your machines.

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