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OSI Layer 3 - The Network Layer

6. Calculating an IPv4 Address in Binary

Each octet in the IP address has a value ranging from zero to 255. Let's see how we get that. So it's an octet. There are eight bits there. Let's count them out. So 112, 34, 5678 bits, and it's binary. So the rightmost digit or the rightmost bit is going to be a one, either a zero or a one. And then we're going to double each time we add a bit to the left. So it goes one, two, 4816, 32, 64, and 128. It's binary, so we multiply by two. We double each time we go up, and the values are going to range from zero to 255 for each octet in our IP address. If we put a zero in on all of those bits, obviously all zeros add up to zero. If we put one in on each of the bets, If you add these up, one, two, eight, plus 64 equals 192. Nine plus two plus 32 equals 224.224 plus 16 equals 204. 254 is 240 plus 52.252 plus two. and 254 plus one is 255. If you're wondering how I was able to add those up so quickly and without even looking like I was thinking about it, you're correct. When you get more experienced with networking, this is going to be second nature to you. IP addressing is really core to everything that you're going to be doing. So you're going to know IP addressing in its native format like the back of your hand, just like I just showed you there. Okay? So that's how we can get the value from zero to 255 on each of those. The bit pattern in here for each of those different bits can be any combination of ones and those. It goes from zero at the lowest value up to two, five, five at the highest value. The example IP address I gave you earlier was 19216 810 15.That's in dotted decimal notation. Let's convert that to binary. So, take a piece of paper and a pencil and write down what you see on the slide. So starting from right, go left, right, out, one, two, 4816, 326-4128. And then to figure out what your IP address is in binary, start off with the first octet: We'll do that, and the first thing to do is to start going from left to right. So when you write it out, it's easiest to write it from right to left. But when you're figuring things out, we go from left to right. So the first thing to ask is, can 128 go into 192? Is 128 equal to or less than 192? And yes, it is. So, if that's the case, we'll put a one under what we do and see what's left, but 192 - 128 equals 64. So we've got 64 left. We then go to the next column and see this. 64 into 64? Yes, it does. So we put another one in, then subtract 64, and we're left with zero. Does 32 go in zero? Obviously not, so it doesn't go away. So we put a zero there, and it's pretty obvious that we've got zero left, so we're going to have zero on all of the rest of the columns, so 192. If we were going to write that in binary, it would be 1100, and then we would do a final check. to ensure that we got it right Add these digits together, so add 128 plus 64. When we add them together, it comes up to 192, which is the value we wanted, so we know that we got that right. When you're working this out, if you ever get anything that does not end with a zero value, you know you've made a mistake, so go back and try it again. Okay, so that was our first octet; let's do the same process again for the second octet of this one-two-eight, which goes into one-six-eight, yes it does.So we'll put a one in there, and then the difference between one six eight and one two eight is four A. We've got four A left in this 64 going to four A No, it doesn't. So we put a zero in that column. Then does 32 go to 4 A? Yes, it does, so we put one in there, and then 40-32 gives us eight left 16 does not go into eight, so we'll put a zero in there. Eight does go into eight, so we put a one in, and eight minus eight gives us zero left, so we know we can just fill in the rest of the columns with a zero; six eight in binary is 101010, and our final check is to add the numbers together: one plus two plus eight plus 32 plus eight does equal six eight, so we got that one right as well. So the first half of our IP address in decimal is 19216; in binary, it's 1100-101-0100, so hopefully you followed along with that. Okay, what I want you to do now is to stop the video and work out the last two octets yourself. So convert 1015 into binary, and when you've done this, I want you to give me the entire IP address in binary notation, so what your output will be is 110-01100 and then the next in binary, and then another dot, and then the final alpha in binary. So go ahead, stop the video now, and we'll check your answer in a second. Okay, so hopefully this is the answer that you got: 19216eight, dot ten, dot 15, so the first octet was 64, which equals one nine two, and the next octet of one six eight was 1010-110-0128 plus 32 plus eight equals one six eight. The next octet you should have got is eight plus two equals ten, and then the last octet of 15 you should have got is 15. If you look at the top of the slide here, that should have been your final output. That is the complete IP address in binary. Hopefully, you find that really simple. With just a tiny bit of practice, you will be able to do this without hardly even really thinking about it. Okay, so that's how we figure out the IP address to set the boundary between our logical networks—our subnet. The IP address is going to be combined with a subnet mask, and you'll learn about the subnet mask in the next lecture.

7. The Subnet Mask

You're going to learn about subnet masks. And you can see what I've done here. I've opened up a command prompt on my Windows laptop again, and eventually IP configuration And you can see where I've highlighted it, that my IP address is 19216 810 15.My subnet mask is 25252 five, and the default gateway is 192-16-8101. So every host in your network is going to know what its IP address, its subnetmask, and its default gateway are. Let's now see how it's going to use that information. So onto the slides. Now, a host can send traffic directly to another host on the same subnet via the switches that they're attached to. But for a host to send traffic to another host in a different subnet, it must be forwarded by a router. So our routers are devices that link our different subnets together and route the traffic between them. The host therefore needs to understand if the destination is on the same or a different subnet in order to know how to send it. If the destination is on the same subnet, it will send it there directly. If it's on a different subnet, it knows that it has to send it to the local router, which is the default gateway. And the way that the host knows whether the destination is on the same subnet or a different subnet is by comparing the IP address of the destination to its own IP address and subnet mask. The subnet mask, just like the IP address, is also 32 bits long. And it can be written in dot-decimal notation, the same as our IP addresses, or it can be written in notation. You'll see how that works a bit later in this lecture. A host IP address is divided into a network portion and a host portion, and it's a subnetmask that defines where the boundary is between the network portion and the host portion of the address. And the easiest way to explain how this works is by giving you an example. Let's say the host IP address is 192.168.10.15 and the subnet mask is 255.254.5255. The IP address and subnet mask are actually on my laptop. To figure this out, we write the IP address out in binary notation, like you learned in the last lecture, and then the subnet mask, also in binary notation, underneath. So our example was 1216810-15, subnetmask 255-255-2550. See the top part here. I've written 19216 810 15 out in binary, and then underneath, 255-255-2550 out in binary as well. The IP address is compared or masked with the subnet mask. A one in the subnet mask indicates that the bit in the IP address is part of the network address, while a zero indicates that it is part of the host address. So very quickly, you can see all the ones on the subnet mask go up to here. Everything in the IP address above that is part of the network portion of the address. The zeros above that in the IP address are part of the host portion of the address. To make this a little bit clearer, so with subnetmask 252–5255, with the subnet mask, it's always going to have contiguous ones. and you see the ones that come up to this part here. So I put a line On that line is the border between the network portion and the host portion of the address. So, in this example, the network address portion is 19216810 because the IP address from here on the left all the way up to the line is 19216 810, which is part of the IP address, and whatever follows the line is the host portion of the address. So in our example, the dot 15 is the host portion of the address. And I've highlighted it. There is the network portion. If the host wants to communicate with another host with an IP address, which also begins with 19216 810 in our example, So say, for example, this host wants to send traffic to a destination address of 19216-810-20. It knows it's on the same subnet, and it can send the traffic directly because the destination also begins with 19216 810.If this host wants to communicate with another host on any of our networks, anything that does not begin with 192-16810, then it knows it has to send the traffic via a router. So if it was sending traffic to destination 19216 81120, for example, it doesn't begin with 19216 810. It's a different subnet that sends it via the router. For a destination address to be on the same subnet, the network portion has to be exactly 19216 810.Anything else means it's a different subnet. We have to go via a router. The subnet mask always begins with a contiguous block of one. This is different than the IP address. You see our example IP address here. It's 110-010-1011. So with the IP address, the ones and the zeros can be mixed up in pretty much any order. The subnet mask is a block of ones and then a block of zeros. In the subnet mask, we never mix the ones in the zerosup with each other. So 111100 is a legal subnet mask. We can't mix up the ones and zeros in 111-0110, one. That is not a valid subnet mask. The host portion of the address is available to be allocated to the different hosts on that particular subnet. For example, your PCs, servers, printers, router interfaces, switch management addresses, et cetera, with two exceptions that you'll see coming up after the next slide. So there's the host portion of the address. For the example highlighted, the host portion of the address specifies the individual host and must be unique on that subnet. Your hosts do not need to be numbered sequentially. For example, we could have a subnet with two hosts on it. One could have addressed ten. The other address could have been 1020. We don't need to number them. Dot one and two. You can't have two different hosts, both with the same IP address. For example, we couldn't have TwoHost's bogus 1010. That would be a duplicate address. And whenever any traffic was sent to 1010, your network devices wouldn't know which host to send it to. So that's illegal. You can't have duplicate IP addresses. You could have host 1010 on one subnet and host 1010 2010 on a different subnet. There are different subnets, so it's not a duplicate address. That's just fine. All zeros in the host portion designate the network address and are not allowed to be allocated to a host. Remember we just said a minute ago that there are two particular addresses that cannot be assigned to a host? The first one of those is all zeros in the host portion that designates the network address or the network ID. In our example, the network address would be 19216 810.Org.So we fill in the bit pattern in the network portion. So the total was 19216 810. And then in the host portion, we put all zeros in there. So with all zeros, you can't assign it to a host. It signifies the network address, which is the bottom address in that particular subnet. And there's one highlighted there; you can see we've used all zeros. The other address, which cannot be assigned to a host, is all at once in the host portion of the address. So all zeros represent the network address, which is the lowest address in the range, and all ones represent the highest address in the range. That is the directed broadcast. Whenever you send traffic to the directed broadcast address, it goes to all hosts in that subnet, not to an individual host. So we can't assign that address to an individual host. And there is a highlighted host portion. I've put all the ones in there. So that leaves 1921-681-0254 in our example available to be allocated to our different hosts. So all the different PCs and other kinds of hosts—maybe we've got some Windows PCs and Linux PCs in that subnet. I can count them from one to 168 one up to one in 2168; ten, two, five, four -- they're all in the same subnet. Whenever they send traffic to each other, they can do that directly without going via their default gateway router.

8. Slash Notation

Yeah, we can also write the subnetmask in slash notation. 255-255-2550 is written in decimal notation. So the example here is 255.255.254.50; we can write that out as a subnet mask. And you see, there is the line between the network portion and the host portion. And as we covered earlier, it's always contiguous ones followed by a block of contiguous zeros. Because it's contiguous, we can count how many there are in a row. So the example here is that we have 24 ones in a row. I know that each of these blocks is an octet. that each equal eight. So that's eight sixty. We can write it like that in decimal notation. Another way we can write it is with a slash, 24. Those two things both mean exactly the same thing. If we had 25525 five-bit netmasks, we could write that as 16 and so on. And whenever you configure a Cisco router or switch on iOS, when you configure the IP address and the subnet mask, you have to write the subnetmask out in the full dot-decimal notation. But whenever we're having a conversation with somebody or if we are creating a network diagram, we will more commonly use slash notation because it's not much fun to see 255-255-5255 all the time. It's much easier to say "24." Also, slash 24 takes up much less space on your network diagrams. So you can make your diagrams look a lot neater and tidier. There was the network portion marked with a slash (24 highlighted.Let's look at another example coming up again.We're looking at the 24 there. So the other example is for IP address 1010 15250.And hopefully, just by looking at this, you're going to see that that's going to be a slash eight. The more you get used to working with IP addresses and subnets, you'll very quickly be able to see where the lines are by just looking at the actual address and mask. So in the example there, the IP address can be written as 10, 10, 15, space two, 5510, 15 eight. when we're writing it out in full with the subnet mask. As an example, the network address is 100 eight. I've put this in bold to highlight it. because the network address is not 10100. When we write this out, it's just the network portion of the address that we specify. As a result, it's a slash eight. So it's only the first octet, which is the network portion. So that's the ten. All the rest of the address, the last three octets, is the host portion. So when we write this address, it would be 100 eight, and the available addresses would be 100 one for our first host. The last host would be ten, 255-255-2654. We can't use ten 255-2525 five, becausethat would be the broadcast address and100 zero is the network address. Okay, that was everything I needed to tell you about the subnet mask for now. But we'll be covering it in more and more detail as we go through the rest of the SEC.

IP Address Classes

1. Introduction

In the last section, we covered the format of an IP address. In this section, you'll learn about the different address classes. So we're going to go through Classes ABC, DE, and E. This information is important to understand before we get into the more advanced topic of subnetting, which we are going to do in the next section.

2. Class A IP Addresses

In this lecture, you'll learn about the IP address classes. We're going to cover Classes A, B, and C here, which are used for allocating addresses to our hosts, and then in the next lecture we will cover Classes D and E. The first thing to talk about is the effect that that line has, which designates where the network portion of the address is and where the host portion of the address is. If a sub-net mask is eight, for example, using the first eight bits for the network portion of the address, then eight bits for the network portion, and 24 bits for the host portion, If we compare that with a slash 24, we're going to have 24 bits for the network portion and only eight bits for the host portion. So we could have a slash eight, which is not going to have many networks but a lot of hosts per network. A slash 24 would have many networks but few hosts per network. So there's a trade-off whenever we decide where to draw the line between how many networks we'll have and how many hosts we'll have. Let's talk about how Internet addressing was originally supposed to work. Because the designers had no idea what would happen with the internet when I, PV Four was first conceived. They didn't realise that there would be a huge explosion of usage, that everybody would be using it, and that everybody would require an IP address. So when they first designed it, they designed it for what was right at that time. And as we go through this section, you need to think about the Internet and how it was then to understand why IPF-4 was designed the way that it is. There are a few issues with IPV4 that you'll learn about as we're going through this section, and those issues came about because the designers didn't realise what was going to happen in the future. So as long as you think about it from that point of view, then everything should make sense. So that was the original way that IP before addressing was meant to work. When a company wanted to communicate on the Internet, they would apply for a range of IP addresses. the global assignment. IP addresses are handled by Ayana. That stands for theInternet Assigned Numbers Authority. So they are up at the top level, looking at it from a global point of view. They assign large blocks of addresses to the local authorities in different regions. As a company, they would apply to their local authority to get a range of public IP addresses. If they had 6000 hosts, for example, they would ask fora range of IP addresses big enough to cover that, plussome room for growth in the future as well. That company would then allocate those addresses to their hosts in their different offices. Unfortunately, when I PV 4 was created, the designers didn't realise how big the Internet was going to get, like we were just saying. So they didn't create a big enough address space. There's not enough IPV Four addresses for everyhost that actually needs an IPV Four addressthat's going to be communicating on the Internet. So IPV Four ran out of addresses. The long-term solution for the problem is IPVSix, which has a much bigger address space. IPV Four is a 32 bit address,IPV Six is a 128 bit address. And we're going to be talking about IPV 6 in a lot of detail in a later section. Private IP addresses with Nat, or networkAddress translation, are currently used as a workaround in the majority of enterprise networks. So IPV 6 is the long-term solution. But today, private addresses are actually more commonly deployed. But as time goes on, there will be more uptake of IPV 6. Again, we're going to discuss private addresses and IPV 6 in a lot more detail as we go through the course. But as I was saying, to understand the next few lectures, don't think about private addressing as an IPV6 yet. We're going to get to there later. to understand why we have those and how they work. You need to understand the original implementation first. So that's what we're going to cover first. So for Class A addresses, the Internet authorities split the global IP address space into separate classes. Class A addresses are assigned to networks with a very large number of hosts. So Class A is going to be a small network portion and a large host portion. The high order bit, which is the first bit in a Class A address, is always set to zero. If you look down at the bottom left of the address there, you'll see I've highlighted the first bit. It's always going to be a zero in a Class A address. The default subnet mask for Class A addresses is 2 and valid network addresses range from 10 to, so that is the network address; the actual host address ranges from 10 625-525-5254, or one to 126. That allows for 126 networks. And if you counted up the host bits, 24 to the power of two adds up to 16,777,214 hosts per network. Now you may have noticed there, if you actually counted this up, the available values if you set the firstbit of zero would actually be we could have allzeros and we could go up to seven. But all zeros and 127 are not in the valid address range that we can assign to our hosts because they are reserved addresses. Zero, zero eight, is reserved and signifies this network, and it's used by some protocols. So 0012, 025-525-5255, are not valid addresses that you can assign to hosts. That entire range is not available. Also, 1278, which is also in the classes, is also reserved. That is used as the loopback address, and it's used for testing the IP stack on the local computer. So 1271 to 1272-552-5255 are not valid host addresses. So you see, as I've written down at the bottom here, "whoops," they just wiped out over 33 million addresses that could have been used for addressing actual hosts on the public Internet. If you think about it, on this network, they could have just used a single address for that, rather than 16 million addresses. And the same for the loop back. They didn't really need 16 million addresses to be used for loopback testing. And if you're thinking, well, there's a huge shortage of addresses on the Internet, why would they do something crazy like that? But the reason is that when I designed PV, they didn't realise that they were going to run out of addresses. So they thought, Yes, no problem, we can assign 16 million addresses for testing. It doesn't matter because we don't have a shortage of addresses. They didn't realise what was going to happen later on. Let me show you how to use the loop back. So let me open up a command prompt here on my laptop. And if I did an IP configuration again, I'd have loads of virtual adapters on here, but if I scroll up slowly, you'll see I don't have the IP address assigned to any of my network cards in here. It's built in with IP, so I canping 12701, which is the loopback address. And you see, I'm getting a reply, which means it's up. The reason you do this is to test that the TCP/IP is working on the local machine. You could ping that server if you were in New York and wanted to make sure you had connectivity to a server in Boston. There's not much point in pinging a server in Boston if you can't get to your local default gateway router in New York. So you would ping that first. And also, there's not much point in pinging the local router if TCP/IP isn't even working on your laptop. So the way that you verify that TCP/IP is up and running on your laptop is by pinging the loop back. We can also call 12701). But like I was just saying, it's the entire class A range beginning with 127 that is actually reserved for testing loop back.So I don't have to pay 127one, I could pay 127 100, 250. And this is going to work as well. So I can ping anything beginning with 127, and it's going to check the local TCP IP stack. So it's good to have that address for testing. not so good that they took out an entire class A network for it. So, obviously, all zeros begin with zero, and one two seven is zero, and all once-class A always begins with a zero as the first bit. Now, obviously, a company that had a class-A address would not put all 16 million hosts into a single logical network. That would be terrible for performance and security. They would split that big eight-range up into smaller subnets and assign those to their different departments in their different offices. For example, if they received the class address 15, they could allocate the subnetwork 150 sales computers in New York. Accounting's fifteen zeros and two zeros were the same as 24 in adecimal notation and sales computers in Boston. So they would have that huge network that they got assigned by the Internet authorities, and they would split it up into smaller subnets that they could assign to their different offices and their different types of hosts in different offices. And when you do that, it's called subnetting. You're going to master subnetting in the next lecture because it's super important.

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