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Cisco Certified Network Associate (CCNA)
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TitleCisco Certified Network Associate (CCNA)
Cisco CCNA Certification Exam Dumps & Practice Test Questions
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Covered the class A addresses. Next up is class B. So class B addresses were originally assigned to medium- to large-sized networks. With class B, the first two bits of the address are always set to 10. Class A addresses default to eight, which is the first octet. Class B subnet masks always default to a16, which is the first two octets. And the valid network addresses range from 12800 T019–12550. This is a class B. The subnet mask is the first two octets. So the ranges use the first two octets. So 1280-219-1255 allows for 16 384 networks and 65 534 hosts on each of those networks. Again, you would never have a flat subnet with up to 650 hosts in there. If you were given a Class B network in the real world, again, you would subnet it into smaller subnets. The last class that we'll cover here, the last one that can be used to assign addresses to hosts, is Class C, and this is used for small networks. You saw that. Class A, we've got the first octet, so eight is used for the subnet mask. Class B, the second octet, is a slash 16. And with class C, the default subnet mask is ASCII 24 up to the end of the third octet. And with class C, it's thefirst three bits that are important. They're always set to bind the 10. The valid network addresses range from 1923–255-2550. That allows for a little over 2 million networks and 254 hosts per network. but something that you're going to know by heart later on. A Class C network has got 254 hosts up to thatamount with a class C that's small enough that that couldbe allocated as is for a real world deployment. Or again, if we wanted to, we could actually subnet that into smaller networks as well. Quick note on private addresses Now, I gave you all the addresses that were available in our class A, B, and C networks, and we spoke about the reserved addresses for our loopbacks and also the addresses beginning with 0 reserved as well. There's also a range of reserved private addresses in each class; those are valid to be assigned to hosts, unlike the other reserved addresses. But the difference is that these ones are not routable on the public internet. So these were originally designed for a host and a closed private network that should have no internet connectivity. For example, imagine you have a high school with students who need to work on PCs but do not want them to be able to browse the internet. You would send them to private addresses. The benefits you get from that are that they can't get hacked because we're not connected to the Internet. And also, it doesn't cost you anything. You have to pay for public IP addresses. Private IP addresses are, of course, free. the different ranges that are used for our private addresses. In class A, it's 100 to 10 and 255-255-5255 on the host addresses. The private host addresses in class B, 170.2160 to 170. 216. 31. Two. Five. Five. And the class CPR atrange 192-1680 to one, 9216-825-5255. Again, when you're experienced with working and networking, these are things that you're going to know off the top of your head. Private addresses: there's quite a bit more we need to talk about with those that are going to be covered in a later lecture in this section. Okay, so that was all the info I needed to give you about our A, B, and C classes for now. We'll talk about the class D andclass E addresses in the next lecture.
In this lecture, you're going to learn about the other address classes. That's Class D and Class E. So you saw in the last lecture that Classes A, B, and C include all the addresses that are valid to be assigned to our end hosts. And those addresses went from one in the first octet to two, then three. But you may be thinking, what about 2240 to 5525-525-5255? Because you know that the maximum value in each of our objects is 255. We only went up to 223. Well, the next class is Class D, and Class D is used for multicast addresses. You'll learn about what multicast is on the next slide, with Class D before high-order bits. The first four bits in the first octet are always set to the binary value of one 10.We can actually count up to ten. We've assigned the bits one and 10 A to the first octet down in the bottom left here. Class D always begins like that. So if we add these up, one two eight plus 64 plus 32 is two two four. So the lowest address is going to be two, two, four. And then the remaining bits over to the right are eight, four, two, and one. If we add those up, it adds up to 15 and two to four, plus 15 adds up to two, three, and nine. So that's where we get the range of two, two-four to two, three-nine from these class D addresses, which were not allocated to hosts, and there was no default subnetmask; they were used just for multicast traffic. So to see how multicast works, let's do a quick review of unicast. First, I've got my centre over on the left, which has got the source IP address 1010. And it's going to send traffic to destinations at 1010 15 and 1010 2015. So it sends traffic to 1010 15.And the source IP address 1010 can be found in the layer 3 IP header of the packet. The destination is 1010 15.And if we're using a Slash 24 subnet mask, they're both in the same subnet, so that traffic can go directly between the hosts without having to go via a router. Let's say that the host then sends traffic to the other host up in the top right. Let's say that this is a video stream that we're sending again, so we've unicast it to 1010 15.We then unicast it to 10:10:2015 as well. If you look in the IP header, again, unicast traffic, the source address is 1010. The destination zip code is 1010, 2015. Again, we were using that 24 subnetmask, so they're on different networks, so the traffic is going to have to go via a router. And if this was video streaming that we were doing here, it's two completely separate pieces of traffic. So it's going to use two megabytes if it was one megabyte per stream. Now, we can improve this by using multi-cast traffic. With multicast traffic, we're going to send one copy of the traffic from 1010, and that one copy is going to get sent to 1010 15 and 1010 2015 as well. We are going to run an application on the sender, which is going to send it as multicast traffic. And in our example, we're going to send it to a destination multicast address of 23901. It still comes from the same source address of 1010, and the destinations still have their normal unicast addresses there as well. But we send it to this special multicast address that will then go to all of the hosts that were interested in getting that traffic. A good analogy for this is that you can think of it like tuning into a radio station. So on those hosts, 1010/15 and 1010/15, they run an application there that says they want to receive the stream for 23901. As long as you've configured support for this on your routers, that traffic will get forwarded to all of the hosts that are interested in receiving it. And the benefit you get is, for example, that we're only sending one meg's worth of bandwidth rather than two meg, and if there were 50 interested hosts, it would still only be one meg's worth of bandwidth rather than 50 meg. So you save a lot on the bandwidth that you're using. It makes things a lot more efficient. Moving on to Class E addresses, they are experimental and reserved for future use. The first bits in a class E are always set to 1111. So again, if we count up, that's going to give us possible values of 240 to two, 5525-525-5255, just like our Class D multicast addresses. These addresses do not have a default subnet mask. There is one special address that is actually used in Class E, which is the broadcast address of two: 5525-525-5255. That is the broadcast address for this network, meaning whatever network the source is on, it's a broadcast for that network. Actually, while we're talking about broadcast traffic, let's just go back to the previous slide for a minute and I'll explain further about why multicast is different and can be more efficient than broadcast traffic. Notice in the example here on the local subnet that the hosts are attached to the switch. It only went to the top host at 1010 15.The traffic did not get sent down to the host below that at the bottom. If this was broadcast traffic, it would be sent to all hosts on that subnet, not just the ones that wanted it. So multicast is more targeted. It's more efficient. Another difference is that as long as you've configured your routers to support it, routers will forward multicast traffic. So that's how we were able to get it to the host up in the top right. Broadcast traffic does not go outside its own local subnet. It does not get forwarded by routers. Okay, so those were our Class D and Class E addresses by default. These Class E Reserve addresses are never really used. You need to know what they are, particularly for the CCNA exam. In the real world, you'll never come across Class E addresses. We're not used in production environments. Class D addresses are used if you're using multicast. Okay, so that was our Class D and our Class E addresses. Let's just have a look at a summary of the different classes before we move on. So Class A is 12126. in the first octet. The default subnet mask is a slash eight. Class B is 128-2191, and that defaults to a slash 16. Class C is 19223, and that defaults to a 24. Classes A, B, and C are the classes that can be assigned to hosts. Class D is for multicast. That uses 224,223 nine. And class E is experimental. That's two four eight to two five. Five. You want to have these classes committed to memory, not just for the CC and A exam, where they're completely essential, but also for real-world networking as well. Okay, that's it. see you in the next one.
The last section, we started off on OSI layer three, and you learned about IP addresses and how we can combine those with the subnet mask to define the boundaries between our logical networks. In this section, we are still going to be on Let's Say Three, but we're going to go much deeper into how we can control the boundaries between our networks with the use of subnetting. Now, if you've heard about this already, you've probably heard that it's one of the more complicated sections to learn from the CCNA. But don't worry, we're going to go through it step by step. I'm going to show you loads of examples. I'm going to get you to work through examples as well. And by the time we're done, you're going to have this down.
This is a short lecture where you're going to learn about Cider, which is classless inter-domain routing. A problem with the original implementation of the classical village was that when the internet authorities gave out addresses, they always gave out a complete class A with an eight, or a complete class B with a 16, or a complete class C with a 24 subnet mask. And this created the problem that if a company had more than 254 hosts that were too big for a class C, they would have to be given a class B. And if they had, say, 500 hosts, they would actually get allocated addresses for 65,534 hosts. So that's obviously way too much. and this led to huge amounts of the global address space being wasted. So Cider classless interdimensional routing was introduced in 1993 as the solution—or at least a partial solution—for that problem. So Cider removed the fixed 816 and 24 requirements for the different address classes and allowed them to be split into smaller networks. And the name for this is "subnetting." Again, we're going to be talking about subnetting a lot in the later lectures as well. So for example, the internet authorities could allocate the address 175 10 20. You can see from the first octet, 175, that that is a class B address, which would normally be 16. But rather than allocating an organisation the entire 16, the internet authorities could now assign them a 20, which meant that other networks in that 175 range would be available to give to other companies. So rather than giving out the huge range, we split the classes into smaller networks that could be given to different organizations, which meant that there would be fewer addresses wasted. So companies can now be allocated a dress range that matches what they actually need and doesn't waste extra addresses. We get another benefit from Cideras as well, which is route summarization. See the example here. We've got ISPA, and they have allocated the address blocks that you see on the left. So one company got one 7510 00:24, another one,one got 7510, 124, one 7510, 224, etc. Up to and including one, 7510, two, five, five. So they've given out 255 address blocks there. We've also got ISPB, and they've given out one 7511, dot, one 7511, one, and so on. Again, at 24, all the way up to 17511, 2-5-5, and ISPA and ISPB get connected. Now, if we didn't have Cider and weren't able to do root summarization, ISPA would advertise that there are actually 256 address blocks there. They would advertise to ISP-B all 256 address blocks, and ISP-B would advertise to ISPA all 256 address blocks. But when we've got Cider and route summarization, what we can do is have the two ISPs advertise just an aggregate block. So ISPA, rather than advertising all 256 24s, advertises 175 10 or 16, which is a superset of all those 256 smaller networks. So, instead of learning 256 routes, ISPB will only learn one route to all networks behind ISPA, and ISPB will only advertise one route of 7511-16 to ISPA. So the benefit we get from this is that ISPs don't know about all 256 networks behind me. It only gets a single summary route that covers all of them. Obviously, if it's got one route rather than256 routes, that's a lot less information. It's more efficient, it takes up less memory in the router, and if an individual link goes down in ISPB, it doesn't have any impact on ISPA because that one summary route doesn't change. It's going to be different in ISPB; whenever one of their routes goes down, the other routes are going to have to recalculate that. But the benefit we get from this is we're compartmentalising the different parts of our network, and if we've got an issue in Part A of the network, it's not going to impact Part B of the network. So it makes things a lot more stable and reliable and makes things more logical, which is better for us humans as well, because it makes it easier to troubleshoot them. Okay, that was Cider, which is closely related to VLSM, our variable lamp subnet masks. We'll be getting to that real soon, too. See you next time.
At the end of this lecture, you're going to understand how we can implement subnetting. Now, I'm actually going to cover subnetting over a few lectures in this section. Subnetting is a really important topic, not just for passing the CCNA exam, but for networking in general. It's really one of the core knowledge areas that you need to understand if you're going to work in networking. So if you need to take a quick break before you watch this lecture, then get yourself a cup of coffee and come back, and we're going to dive deep into networking. To understand this lecture, you need to think about it from the context of the originally intended IPV4 design, just like we did in the previous few lectures there, where all hosts that can communicate on the Internet have a public IP address. Now, if you're already working in a company where you've already had exposure to working on the network, chances are your company is going to be using private IP addresses. Now, as we go through these lectures, do not think about it from that point of view, okay? We're going to get into private IP addresses later, and I'll also explain how modern-day networks work. But before we get there, you need to understand how IPV Four worked in the first place. It's a natural progression. Understand this first, and then you'll be able to understand why we use private addresses on our networks and how they work as well. Okay? So just bear with me and think about it from the point of view of how the Internet worked, like 10 or 15 years ago, before we were running out of addresses and before everybody was using private addresses. While I say that, by the way, the majority of companies are using private addresses, but there are a few out there that are using public addresses everywhere. That is pretty rare, though. Okay, so for example, let's say that we're a network designer for a small business with four apartments that are spread over two offices, and we want to manage our own public address space. So what we could do is buy four separate Class C networks for that. But the problem is that public IP addresses cost money. So rather than doing that, rather than buying separate networks from the Internet authorities, we could purchase a single range and subnet it into smaller portions. Let's say we've only got a handful of hosts in each of those departments. Rather than buying 256 addresses for each, we can buy a single Class C range, and then we can divide that network up into smaller networks and assign it to the different parts of our networks. As an example, suppose we have that Class C range and the default class subnet masks are 215 to 00:24. So we have our subnet. To divide that network into smaller subnets, we need to burrow some host bits and add them to the network portion of the address. So we're going to move the line that separates the network portion of the address and the host portion further over to the right. So we're going to take some of our host addresses away and give them to the network portion of the address. When we do subnet, the line always moves to the right, and the further to the right we go, the more subnets that we're going to have, but the fewer hosts that we'll have on each subnet. To calculate the number of available subnets that we're going to get by moving the line to the right, we use a formula of two to the power of subnet bits. When I first saw this, I thought to myself, "Oh, I don't know how to do this; we never really covered that in school." Don't worry, it's super simple, as you'll see as we go through these next few lectures. So if a class C network uses a slash 28 subnetmask, then we've borrowed four bits from the default of 24. The normal class C mask is a slash 24. If we change that to a slash 28, the difference between 24 and 28 is for how we calculate how many subnets we'll get as a result, which is two times two to the power of the number of subnet bits borrowed. So we borrowed four bits from the network portion of the host portion to see how many networks we're going to get. It's two to the power of four to calculate. This is dead easy. You can do it on your fingers. You start off with two, and you just double. So I go to 4816. I'm going to have 16 available subnets. Another example: if a class B network uses the same 28-subnet mask, the default for a class B is 16. So if we subnet that to a slash 28, we've borrowed twelve bits to see how many subnets we are going to get. It is going to be two to the power of twelve. So let's do this. One, two, 48, 16, 32, 64, 128, 256, 512, 10, 24, 20, 48, 40, 96 subnets for that one. You might have noticed that it's easy if you remember that to the power of ten is 1024. We just keep doubling each time. And remember that hosts on different subnets need to go via a router if they want to communicate with each other. That's the whole point of having our IP addressing and of doing subnetting. It's to divide our network up into different logical parts of the network, and it's the routers that are the devices that know how to get everywhere and that can direct the traffic. So that was how to calculate the number of subnets we're going to get. It's two to the power of how many bits we borrowed to calculate the number of hosts. It's two times how many host bits there are, minus two. We need to subtract two because we've got the network address and the broadcast address that cannot be assigned to hosts. That's what we take away. So if a class C network uses a slash 28 subnet mask, then we've got four bits left for host. There are 32 bits in the address. If we're using 28 for the network portion, that leaves four for the host portion. Two to the power of four is two, 4816 minus two, for the network address and the broadcast address. That means that we would have 14 available addresses for our hosts. If we've got a class B network and it's using ASCII 28 again, we've got four bits left for our hosts. So two of 4816 minus two is 14. It's exactly the same. So keep in mind that the number of hosts we'll get is going to be determined by the subnet size, specifically the subnet mass size. It's going to be the same whether it's Class A, Class B, or Class C. However, the number of subnets I'll get if I go back a slide will differ between classes A and B due to different default subnet mask sizes. For example, if we use a slash 28 with a class B, then we're going to have 4096 available subnets. If we use a slash 28 with a classC, we're going to have 16 available subnets. There's a difference there, but for the host, it's always going to be the same. A quick note on the Ipsomenet zero command Just like we have to subtract two to get the number of valid hosts, back in the day, we also had to subtract two from the number of available networks. This is because in the original Internet standards, it wasn't allowed to use network beds of all zeros or all ones, just like we can't use host beds of all zeros or all ones. So that took away two of our available subnets. However, there wasn't really any practical need for that, and it wasted a dress base. But it's a practical need with the hostbets because we've got that network address and the broadcast address that are actually used. But as far as the number of subnets goes, we were taking away two, not really for any good reason. So, for quite some time, there has been a default command of IPsubnet zero on Cisco routers that disables that behavior. The command is enabled by default. So those two extra network addresses are available on Cisco networks. It's important that I tell you this because you might look up something on the Internet; you might look up a subnet calculator there, and it will tell you that there are 14 networks available on a class C with a 28. For example, when I tell you that there are 16 Cisco networks, do not take too much away from the number of available subnets. And I want to tell you that just so you don't get confused if you're looking up anything on the Internet.
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