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Dialogue: 0,0:00:01.14,0:00:07.23,Default,,0000,0000,0000,,Welcome to Jeremy's IT Lab. This is a free, complete course for the CCNA. If you like
Dialogue: 0,0:00:07.23,0:00:12.67,Default,,0000,0000,0000,,these videos, please subscribe to follow along with the series. Also, please like, leave
Dialogue: 0,0:00:12.67,0:00:15.67,Default,,0000,0000,0000,,a comment, and share the video to help spread this free series of videos.
Dialogue: 0,0:00:15.67,0:00:17.65,Default,,0000,0000,0000,,Thanks for your help.
Dialogue: 0,0:00:18.77,0:00:24.75,Default,,0000,0000,0000,,In this video, we will be talking about subnetting. This is a very big topic for the CCNA, but
Dialogue: 0,0:00:24.75,0:00:29.90,Default,,0000,0000,0000,,not just for the test--it’s an essential skill for a network engineer. Many people
Dialogue: 0,0:00:29.90,0:00:35.59,Default,,0000,0000,0000,,have trouble understanding subnetting, but let me assure you, it is not difficult. Subnetting
Dialogue: 0,0:00:35.59,0:00:41.81,Default,,0000,0000,0000,,is very simple if you take it step by step. So, I’m going to split subnetting into two,
Dialogue: 0,0:00:41.81,0:00:46.88,Default,,0000,0000,0000,,or maybe even three, videos so we can take our time to really understand subnetting without
Dialogue: 0,0:00:46.88,0:00:52.61,Default,,0000,0000,0000,,getting lost. Now, because subnetting is such an important topic, and many people have trouble
Dialogue: 0,0:00:52.61,0:00:58.26,Default,,0000,0000,0000,,with it, there are already plenty of subnetting videos on YouTube. Of course, feel free to
Dialogue: 0,0:00:58.26,0:01:02.92,Default,,0000,0000,0000,,check out those videos too--there are some different tricks and techniques people teach
Dialogue: 0,0:01:02.92,0:01:08.21,Default,,0000,0000,0000,,that can speed up the subnetting process. I, however, will simply outline the basic
Dialogue: 0,0:01:08.21,0:01:14.31,Default,,0000,0000,0000,,steps involved in subnetting. I will avoid overcomplicating the topic. My end
Dialogue: 0,0:01:14.31,0:01:20.43,Default,,0000,0000,0000,,goal for these videos is that you understand and can do subnetting. So, let’s get started.
Dialogue: 0,0:01:20.43,0:01:28.02,Default,,0000,0000,0000,,So, what will we cover in this video? Just a couple of things. First is CIDR, pronounced
Dialogue: 0,0:01:28.02,0:01:35.42,Default,,0000,0000,0000,,“CIDR,” which stands for Classless Inter-Domain Routing. What exactly is that? Well, remember
Dialogue: 0,0:01:35.42,0:01:42.61,Default,,0000,0000,0000,,I introduced the IPv4 address classes, such as Class A, B, and C? Well, CIDR throws all
Dialogue: 0,0:01:42.61,0:01:48.75,Default,,0000,0000,0000,,that away and lets us be more flexible with our IPv4 networks. Then, of course, we’ll
Dialogue: 0,0:01:48.75,0:01:54.37,Default,,0000,0000,0000,,cover the process of subnetting, taking it step by step so you don’t get lost.
Dialogue: 0,0:01:54.37,0:02:00.01,Default,,0000,0000,0000,,Now, before I get into CIDR, let’s review these IPv4 address classes so we can then
Dialogue: 0,0:02:00.01,0:02:06.91,Default,,0000,0000,0000,,understand the need for classless IPv4 addressing. There are five classes of IPv4 addresses:
Dialogue: 0,0:02:06.91,0:02:14.80,Default,,0000,0000,0000,,A, B, C, D, and E. Class A addresses have a first octet beginning with zero, and the rest
Dialogue: 0,0:02:14.80,0:02:20.48,Default,,0000,0000,0000,,of the bits can either be zero or one. This leads to a decimal range for the first octet of
Dialogue: 0,0:02:20.48,0:02:29.25,Default,,0000,0000,0000,,0 to 127. Remember, an IPv4 address is 32 bits, so there are 4 octets--4 groups of 8
Dialogue: 0,0:02:29.25,0:02:42.18,Default,,0000,0000,0000,,bits--in an IPv4 address. This makes the Class A address range from 0.0.0.0 through 127.255.255.255.
Dialogue: 0,0:02:42.18,0:02:46.96,Default,,0000,0000,0000,,Now, remember, there are some special and reserved addresses in these ranges that can’t be
Dialogue: 0,0:02:46.96,0:02:52.06,Default,,0000,0000,0000,,used for normal IP addresses on a device, but for this video, we’ll just include all
Dialogue: 0,0:02:52.06,0:02:59.51,Default,,0000,0000,0000,,of them in Class A. Class B addresses have a first octet beginning with 10, and the
Dialogue: 0,0:02:59.51,0:03:06.56,Default,,0000,0000,0000,,other 6 bits can be either 0 or 1. This gives a range for the first octet of 128 through
Dialogue: 0,0:03:06.56,0:03:18.75,Default,,0000,0000,0000,,191. The address range for Class B is 128.0.0.0 through 191.255.255.255. Class C addresses
Dialogue: 0,0:03:18.75,0:03:25.02,Default,,0000,0000,0000,,have the first three bits set to 110, and the others can be either zero or one. If you write
Dialogue: 0,0:03:25.02,0:03:33.53,Default,,0000,0000,0000,,that range in decimal, it is 192 through 223. The address range is therefore 192.0.0.0
Dialogue: 0,0:03:33.53,0:03:42.75,Default,,0000,0000,0000,,through 223.255.255.255. Class D addresses begin with 1110 in binary, which gives
Dialogue: 0,0:03:42.75,0:03:49.71,Default,,0000,0000,0000,,a range of 224 through 239 for the first octet of the address. This means that the address range
Dialogue: 0,0:03:49.71,0:04:02.00,Default,,0000,0000,0000,,for Class D is 224.0.0.0 through 239.255.255.255. Finally, Class E addresses begin with 1111
Dialogue: 0,0:04:02.00,0:04:11.47,Default,,0000,0000,0000,,in binary, so the first octet range is 240 through 255, and therefore the address range is 240.0.0.0
Dialogue: 0,0:04:11.47,0:04:14.09,Default,,0000,0000,0000,,through 255.255.255.255.
Dialogue: 0,0:04:14.09,0:04:21.97,Default,,0000,0000,0000,,However, only the Class A, B, and C addresses can be assigned to a device as an IP address,
Dialogue: 0,0:04:21.97,0:04:28.68,Default,,0000,0000,0000,,as Classes D and E have special purposes, as I mentioned in the IPv4 addressing videos. Class
Dialogue: 0,0:04:28.68,0:04:34.55,Default,,0000,0000,0000,,A addresses have an 8-bit prefix length, meaning the first octet identifies the network and
Dialogue: 0,0:04:34.55,0:04:40.20,Default,,0000,0000,0000,,the other three octets are used for individual hosts within the network. Class B addresses
Dialogue: 0,0:04:40.20,0:04:45.53,Default,,0000,0000,0000,,have a 16-bit prefix length, so the first two octets identify the network, and the last
Dialogue: 0,0:04:45.53,0:04:51.61,Default,,0000,0000,0000,,two octets identify individual hosts within that network. Class C addresses have a prefix
Dialogue: 0,0:04:51.61,0:04:57.60,Default,,0000,0000,0000,,length of 24, so the first three octets are used to identify the network, and only the
Dialogue: 0,0:04:57.60,0:05:03.34,Default,,0000,0000,0000,,last octet is used to identify individual hosts within that network.
Dialogue: 0,0:05:03.34,0:05:08.28,Default,,0000,0000,0000,,The different prefix lengths give different characteristics to these classes. As you can
Dialogue: 0,0:05:08.28,0:05:14.13,Default,,0000,0000,0000,,see, there are few Class A networks available--only 128, actually less than that because
Dialogue: 0,0:05:14.13,0:05:21.28,Default,,0000,0000,0000,,some are reserved, like the 127.0.0.0 range, which you may remember is used for loopback
Dialogue: 0,0:05:21.28,0:05:26.87,Default,,0000,0000,0000,,addresses. Because only the first octet of a Class A address is used for the network ID,
Dialogue: 0,0:05:26.87,0:05:32.04,Default,,0000,0000,0000,,there are three whole octets available for addresses within each Class A network,
Dialogue: 0,0:05:32.04,0:05:40.41,Default,,0000,0000,0000,,so there are 16,777,216 addresses in each Class A network. That is
Dialogue: 0,0:05:40.41,0:05:48.22,Default,,0000,0000,0000,,2 to the power of 24, because there are three octets (3 times 8 = 24 bits). Class B
Dialogue: 0,0:05:48.22,0:05:55.27,Default,,0000,0000,0000,,addresses are different. There are more Class B networks--16,384--but fewer addresses per
Dialogue: 0,0:05:55.27,0:06:03.25,Default,,0000,0000,0000,,network, 65,536, which is still many addresses, of course. Finally, there are very
Dialogue: 0,0:06:03.25,0:06:12.85,Default,,0000,0000,0000,,many Class C networks--2,097,152 networks--but only 256 addresses per network.
Dialogue: 0,0:06:12.85,0:06:19.85,Default,,0000,0000,0000,,So, how does a company get their own network address range to use? Well, IP addresses are assigned to
Dialogue: 0,0:06:19.85,0:06:26.25,Default,,0000,0000,0000,,companies or organizations by a nonprofit American corporation called the IANA, the
Dialogue: 0,0:06:26.25,0:06:32.85,Default,,0000,0000,0000,,Internet Assigned Numbers Authority. The IANA assigns IPv4 addresses and networks to companies
Dialogue: 0,0:06:32.85,0:06:39.16,Default,,0000,0000,0000,,based on their size. For example, a very large company might receive a Class A or Class B
Dialogue: 0,0:06:39.16,0:06:44.37,Default,,0000,0000,0000,,network. Remember, there are lots of available addresses to use for hosts in each Class A
Dialogue: 0,0:06:44.37,0:06:49.91,Default,,0000,0000,0000,,and Class B network. While a small company might receive a Class C network, because there
Dialogue: 0,0:06:49.91,0:06:56.81,Default,,0000,0000,0000,,are fewer addresses in each Class C network--only 256. However, this system led to many
Dialogue: 0,0:06:56.81,0:07:02.88,Default,,0000,0000,0000,,wasted IP addresses, so multiple methods of improving this system have been created. Let
Dialogue: 0,0:07:02.88,0:07:08.68,Default,,0000,0000,0000,,me give you an example of how this strict system of addresses can waste IP addresses.
Dialogue: 0,0:07:08.68,0:07:15.82,Default,,0000,0000,0000,,So, here are two routers. As you can see, R1 has three networks connected to it here.
Dialogue: 0,0:07:15.82,0:07:20.91,Default,,0000,0000,0000,,Remember that routers are used to connect different networks, so each of these links is a separate
Dialogue: 0,0:07:20.91,0:07:28.28,Default,,0000,0000,0000,,Layer 3 network, different IP networks. R2 also has three networks connected here. Perhaps
Dialogue: 0,0:07:28.28,0:07:33.30,Default,,0000,0000,0000,,each of these networks will have a few switches, with many end hosts such as PCs and servers
Dialogue: 0,0:07:33.30,0:07:39.65,Default,,0000,0000,0000,,connected to these switches. However, there is one more network here. That’s this network
Dialogue: 0,0:07:39.65,0:07:45.29,Default,,0000,0000,0000,,connecting these two routers. This is known as a point-to-point network, meaning
Dialogue: 0,0:07:45.29,0:07:51.59,Default,,0000,0000,0000,,that it’s a network connecting two points, in this case, R1 and R2. For example, this
Dialogue: 0,0:07:51.59,0:07:57.57,Default,,0000,0000,0000,,might be a connection between offices in different cities, let’s say San Francisco and New York.
Dialogue: 0,0:07:58.54,0:08:04.90,Default,,0000,0000,0000,,So, because this is a point-to-point network, we don’t need a large address block, so
Dialogue: 0,0:08:04.90,0:08:14.02,Default,,0000,0000,0000,,let’s use a Class C network, 203.0.113.4. Because this is a Class C network, there are
Dialogue: 0,0:08:14.02,0:08:22.62,Default,,0000,0000,0000,,256 addresses in the network, minus one for the network address (203.0.113.0), minus one
Dialogue: 0,0:08:22.62,0:08:30.74,Default,,0000,0000,0000,,for the broadcast address (203.0.113.255), minus one for R1’s address, which I’ll
Dialogue: 0,0:08:30.74,0:08:40.30,Default,,0000,0000,0000,,assign as 203.0.113.1, and minus one for R2’s address, which I’ll assign as 203.0.113.2.
Dialogue: 0,0:08:40.30,0:08:45.33,Default,,0000,0000,0000,,That’s a total of four addresses used and 252 addresses wasted.
Dialogue: 0,0:08:45.33,0:08:48.73,Default,,0000,0000,0000,,Clearly, this is not an ideal system.
Dialogue: 0,0:08:50.84,0:08:56.98,Default,,0000,0000,0000,,Before introducing CIDR, here’s another quick example of address waste. A company,
Dialogue: 0,0:08:56.98,0:09:05.31,Default,,0000,0000,0000,,Company X, needs IP addressing for 5,000 end hosts. This is a problem, why? A Class C network
Dialogue: 0,0:09:05.31,0:09:11.14,Default,,0000,0000,0000,,does not provide enough addresses, so a Class B network must be assigned. Because a Class
Dialogue: 0,0:09:11.14,0:09:18.02,Default,,0000,0000,0000,,B network allows for about 65,000 addresses, this results in about 60,000 addresses being wasted.
Dialogue: 0,0:09:19.61,0:09:23.97,Default,,0000,0000,0000,,When the Internet was first created, the creators did not predict that the Internet would become
Dialogue: 0,0:09:23.97,0:09:30.15,Default,,0000,0000,0000,,as large as it is today. This resulted in wasted address space, like the examples I showed
Dialogue: 0,0:09:30.15,0:09:35.89,Default,,0000,0000,0000,,you, and there are many more examples that I could show you. The total IPv4 address space
Dialogue: 0,0:09:35.89,0:09:40.97,Default,,0000,0000,0000,,includes over 4 billion addresses, and that seemed like a huge number of addresses when
Dialogue: 0,0:09:40.97,0:09:52.06,Default,,0000,0000,0000,,IPv4 was created, but now address space exhaustion is a big problem. There's not enough addresses. One way to solve, or remedy, this problem is
Dialogue: 0,0:09:52.06,0:10:00.19,Default,,0000,0000,0000,,CIDR. The IETF (Internet Engineering Task Force) introduced CIDR in 1993 to replace
Dialogue: 0,0:10:00.19,0:10:02.11,Default,,0000,0000,0000,,the classful addressing system.
Dialogue: 0,0:10:03.76,0:10:10.31,Default,,0000,0000,0000,,With CIDR, the requirements of Class A addresses to use an 8-bit network mask, Class
Dialogue: 0,0:10:10.31,0:10:18.19,Default,,0000,0000,0000,,B to use 16, and Class C to use 24 were removed. This allowed larger networks
Dialogue: 0,0:10:18.19,0:10:24.27,Default,,0000,0000,0000,,to be split into smaller networks, allowing greater efficiency. These smaller networks
Dialogue: 0,0:10:24.27,0:10:29.34,Default,,0000,0000,0000,,are called subnetworks, or subnets. Let’s look at an example of splitting a
Dialogue: 0,0:10:29.34,0:10:33.99,Default,,0000,0000,0000,,larger network into a smaller network so you can see how it works.
Dialogue: 0,0:10:33.99,0:10:40.78,Default,,0000,0000,0000,,Here’s the same point-to-point network we looked at before. Previously, it was assigned
Dialogue: 0,0:10:40.78,0:10:48.64,Default,,0000,0000,0000,,the 203.0.113.0/24 network space, but that resulted in lots of wasted addresses. Let’s
Dialogue: 0,0:10:48.64,0:10:55.83,Default,,0000,0000,0000,,write this out in binary. Here’s the binary, with the dotted decimal underneath. Now, the
Dialogue: 0,0:10:55.83,0:11:04.68,Default,,0000,0000,0000,,prefix length is 24, so here’s the network mask, also known as the subnet mask: 255.255.255.0.
Dialogue: 0,0:11:04.68,0:11:11.21,Default,,0000,0000,0000,,Remember, all 1s in the subnet mask indicate that the same bit in the address
Dialogue: 0,0:11:11.21,0:11:16.85,Default,,0000,0000,0000,,is the network portion. In this case, I’ve made the network portion blue, and the host portion
Dialogue: 0,0:11:16.85,0:11:26.38,Default,,0000,0000,0000,,is red. Well, how many host bits are there? 8, because it’s one octet. So, how many potential hosts, or how
Dialogue: 0,0:11:26.38,0:11:33.52,Default,,0000,0000,0000,,many usable addresses, are there? Well, the formula is this: 2 to the power of 8 minus
Dialogue: 0,0:11:33.52,0:11:41.83,Default,,0000,0000,0000,,2 equals 254 usable addresses. What is the 8? Well, it’s the number of host bits, which is
Dialogue: 0,0:11:41.83,0:11:48.84,Default,,0000,0000,0000,,8 in this case. And why minus 2? Those are the network address and the broadcast address.
Dialogue: 0,0:11:48.84,0:11:53.76,Default,,0000,0000,0000,,We can’t assign them to a device, so we have to remove them from the number of usable addresses.
Dialogue: 0,0:11:53.76,0:12:01.56,Default,,0000,0000,0000,,So, we have 254 usable addresses, but we only need two--one for R1 and one for R2.
Dialogue: 0,0:12:01.56,0:12:09.20,Default,,0000,0000,0000,,However, CIDR allows us to use different prefix lengths, so it doesn’t have to be 24.
Dialogue: 0,0:12:09.20,0:12:13.86,Default,,0000,0000,0000,,Let’s get some practice calculating the number of hosts within different prefix lengths.
Dialogue: 0,0:12:13.86,0:12:34.21,Default,,0000,0000,0000,,203.0.113.0/25, 203.0.113.0/26, 203.0.113.0/27, /28, /29, /30, /31, and finally /32. I’ve
Dialogue: 0,0:12:34.21,0:12:39.73,Default,,0000,0000,0000,,put /31 and /32 in red because they’re a little bit special, as you’ll see when you
Dialogue: 0,0:12:39.73,0:12:46.12,Default,,0000,0000,0000,,try to calculate it. So, pause the video here and try to calculate how many usable addresses
Dialogue: 0,0:12:46.12,0:12:53.85,Default,,0000,0000,0000,,are on each network. Okay, let’s check out the answers.
Dialogue: 0,0:12:53.85,0:13:01.72,Default,,0000,0000,0000,,So, here is 203.0.113.0/25, but this time with a /25 mask. Notice that the network portion
Dialogue: 0,0:13:01.72,0:13:06.57,Default,,0000,0000,0000,,of the address has extended into the first bit of the last octet, and the mask
Dialogue: 0,0:13:06.57,0:13:15.91,Default,,0000,0000,0000,,in dotted decimal is now written as 255.255.255.128. I changed the color of the extra bit to purple,
Dialogue: 0,0:13:15.91,0:13:21.14,Default,,0000,0000,0000,,but it is part of the network portion, which is the blue part. If you don’t remember how to convert
Dialogue: 0,0:13:21.14,0:13:27.13,Default,,0000,0000,0000,,from binary to dotted decimal, make sure you review that; it’s very important for subnetting.
Dialogue: 0,0:13:27.13,0:13:31.52,Default,,0000,0000,0000,,Now, there are 7 bits in the host portion of the address, so the number of usable addresses
Dialogue: 0,0:13:31.52,0:13:40.28,Default,,0000,0000,0000,,is 2 to the power of 7 minus 2, which equals 126. Once again, we only need two addresses--
Dialogue: 0,0:13:40.28,0:13:47.29,Default,,0000,0000,0000,,one for R1 and one for R2--so we will be wasting 124 addresses. That’s better than wasting
Dialogue: 0,0:13:47.29,0:13:53.89,Default,,0000,0000,0000,,252 addresses with a /24 prefix length, but it’s still wasteful.
Dialogue: 0,0:13:53.89,0:14:02.16,Default,,0000,0000,0000,,How about a /26 prefix length? Notice that it’s now written as 255.255.255.192 in dotted
Dialogue: 0,0:14:02.16,0:14:08.35,Default,,0000,0000,0000,,decimal, because two bits of the last octet are now part of the network portion. Since
Dialogue: 0,0:14:08.35,0:14:14.94,Default,,0000,0000,0000,,there are six host bits, there are now 62 usable addresses in this network. If we were to use
Dialogue: 0,0:14:14.94,0:14:23.64,Default,,0000,0000,0000,,a /26 network mask for the 203.0.113.0 network, we would be wasting 60 addresses. Getting
Dialogue: 0,0:14:23.64,0:14:27.73,Default,,0000,0000,0000,,better, but we can make this network even smaller.
Dialogue: 0,0:14:27.73,0:14:33.69,Default,,0000,0000,0000,,Now that you get the idea, let’s speed it up. For a /27 prefix length, the mask is written
Dialogue: 0,0:14:33.69,0:14:42.66,Default,,0000,0000,0000,,as 255.255.255.224 in dotted decimal. There are now five host bits, so that means there are
Dialogue: 0,0:14:42.66,0:14:50.39,Default,,0000,0000,0000,,30 usable addresses. As you can see, the address space is getting smaller and smaller as we extend the network mask.
Dialogue: 0,0:14:50.65,0:15:00.92,Default,,0000,0000,0000,,For a /28 prefix length, the mask is written as 255.255.255.240 in dotted decimal. There
Dialogue: 0,0:15:00.92,0:15:08.18,Default,,0000,0000,0000,,are now only four host bits, so that means there are 14 usable addresses. After assigning addresses
Dialogue: 0,0:15:08.18,0:15:14.50,Default,,0000,0000,0000,,to R1 and R2, this would mean only 12 wasted addresses, but we can make this address space
Dialogue: 0,0:15:14.50,0:15:19.68,Default,,0000,0000,0000,,even smaller to make our addressing even more efficient.
Dialogue: 0,0:15:19.68,0:15:28.33,Default,,0000,0000,0000,,If we use a /29 prefix length, the mask is written as 255.255.255.248 in dotted decimal.
Dialogue: 0,0:15:28.33,0:15:34.92,Default,,0000,0000,0000,,Now we have only three host bits, so that means there are just six usable addresses. Again,
Dialogue: 0,0:15:34.92,0:15:41.51,Default,,0000,0000,0000,,after we give R1 and R2 addresses, there would be only four wasted addresses.
Dialogue: 0,0:15:41.51,0:15:50.14,Default,,0000,0000,0000,,If we use a /30 prefix length, the mask is written as 255.255.255.252 in dotted decimal.
Dialogue: 0,0:15:50.14,0:15:56.92,Default,,0000,0000,0000,,There are now only two host bits, so that means two usable addresses. So, this is perfect. There
Dialogue: 0,0:15:56.92,0:16:03.06,Default,,0000,0000,0000,,are four total addresses: the network address, the broadcast address, R1’s address, and
Dialogue: 0,0:16:03.06,0:16:07.90,Default,,0000,0000,0000,,R2’s address. That means zero wasted addresses.
Dialogue: 0,0:16:07.90,0:16:19.15,Default,,0000,0000,0000,,Before moving on to look at the /31 and /32 prefix lengths, let me clarify a little bit. So, instead of 203.0.113.0/24,
Dialogue: 0,0:16:19.15,0:16:30.17,Default,,0000,0000,0000,,we will use 203.0.113.0, which is a subnet of that larger Class C network. 203.0.113.0
Dialogue: 0,0:16:30.17,0:16:38.09,Default,,0000,0000,0000,,includes the address range of 203.0.113.0 through 203.0.113.3. Let me show you that
Dialogue: 0,0:16:38.09,0:16:50.42,Default,,0000,0000,0000,,in binary. Here is 203.0.113.0 in binary, the host portion all zeroes. Here is 203.0.113.1,
Dialogue: 0,0:16:50.42,0:17:00.38,Default,,0000,0000,0000,,203.0.113.2, and 203.0.113.3. These are the four addresses in the network, with these two being
Dialogue: 0,0:17:00.38,0:17:07.35,Default,,0000,0000,0000,,the two usable addresses, which are assigned to R1 and R2. So, we took up four addresses with
Dialogue: 0,0:17:07.35,0:17:14.88,Default,,0000,0000,0000,,this subnet. What about the other addresses in the 203.0.113.0/24 range? The remaining
Dialogue: 0,0:17:14.88,0:17:24.71,Default,,0000,0000,0000,,addresses in the address block, which are 203.0.113.4 through 203.0.113.255, are now available
Dialogue: 0,0:17:24.71,0:17:33.56,Default,,0000,0000,0000,,to be used in other subnets. That’s the magic of subnetting. Instead of using 203.0.113.0/24
Dialogue: 0,0:17:33.56,0:17:42.48,Default,,0000,0000,0000,,and wasting 252 addresses, we can use /30 and waste no addresses. Or, perhaps there is another
Dialogue: 0,0:17:42.48,0:17:47.34,Default,,0000,0000,0000,,way to make this even more efficient. Let’s look into it.
Dialogue: 0,0:17:47.34,0:17:57.01,Default,,0000,0000,0000,,If we use a /31 prefix length, the mask is written as 255.255.255.254 in dotted decimal.
Dialogue: 0,0:17:57.01,0:18:06.10,Default,,0000,0000,0000,,There is now only one host bit, so that means zero usable addresses. Two to the power of one is two,
Dialogue: 0,0:18:06.10,0:18:12.03,Default,,0000,0000,0000,,minus two for the network and broadcast addresses, means zero addresses that we can assign to devices.
Dialogue: 0,0:18:12.03,0:18:20.35,Default,,0000,0000,0000,,So, you used to not be able to use /31 network prefixes because of this. However, for a point-to-point
Dialogue: 0,0:18:20.35,0:18:26.07,Default,,0000,0000,0000,,connection like this, it actually is possible to use a /31 mask. Let's check it out.
Dialogue: 0,0:18:27.46,0:18:41.18,Default,,0000,0000,0000,,So, here’s the 203.0.113.0/31 network. R1 is 203.0.113.0, and R2 is 203.0.113.1. The
Dialogue: 0,0:18:41.18,0:18:50.74,Default,,0000,0000,0000,,203.0.113.0/31 network consists of addresses from 203.0.113.0 through 203.0.113.1, which
Dialogue: 0,0:18:50.74,0:18:58.58,Default,,0000,0000,0000,,is actually only two addresses. Here they are in binary. There’s 203.0.113.0, and
Dialogue: 0,0:18:58.58,0:19:05.56,Default,,0000,0000,0000,,there’s 203.0.113.1. Normally, this would be a problem because it leaves no usable
Dialogue: 0,0:19:05.56,0:19:10.24,Default,,0000,0000,0000,,addresses after subtracting the network and broadcast addresses, but for point-to-point
Dialogue: 0,0:19:10.24,0:19:15.53,Default,,0000,0000,0000,,networks like this, a dedicated connection like this between two routers, there is actually
Dialogue: 0,0:19:15.53,0:19:20.90,Default,,0000,0000,0000,,no need for a network address or a broadcast address. So, we can break the rules in this
Dialogue: 0,0:19:20.90,0:19:27.29,Default,,0000,0000,0000,,case and assign the only two addresses in this network to our routers. Note that if
Dialogue: 0,0:19:27.29,0:19:32.02,Default,,0000,0000,0000,,you try this configuration on a Cisco router, you’ll get a warning like this, reminding
Dialogue: 0,0:19:32.02,0:19:37.49,Default,,0000,0000,0000,,you to make sure that this is a point-to-point link, but it is a totally valid configuration.
Dialogue: 0,0:19:37.49,0:19:49.56,Default,,0000,0000,0000,,So, once again, the remaining addresses in the 203.0.113.0/24 address block, which are 203.0.113.2 through 203.0.113.255,
Dialogue: 0,0:19:49.56,0:19:54.73,Default,,0000,0000,0000,,are now available to be used in other networks. But this time, we've
Dialogue: 0,0:19:54.73,0:20:02.11,Default,,0000,0000,0000,,saved even more addresses, using only two addresses instead of four for this point-to-point connection.
Dialogue: 0,0:20:02.11,0:20:08.13,Default,,0000,0000,0000,,People still do use 30 for point-to-point connections at times, but 31 masks are totally
Dialogue: 0,0:20:08.13,0:20:12.38,Default,,0000,0000,0000,,valid and more efficient than 30, so I recommend this method.
Dialogue: 0,0:20:14.13,0:20:23.03,Default,,0000,0000,0000,,But, we still haven't looked at the 32 mask. A 32 mask is written as 255.255.255.255 in
Dialogue: 0,0:20:23.03,0:20:29.62,Default,,0000,0000,0000,,dotted decimal, making the entire address the network portion. There are no host bits.
Dialogue: 0,0:20:29.62,0:20:35.28,Default,,0000,0000,0000,,If you calculate this using our formula, you will get one usable address. Clearly, the
Dialogue: 0,0:20:35.28,0:20:41.51,Default,,0000,0000,0000,,formula doesn't work in this case. You won't be able to use a 32 mask in this case, and
Dialogue: 0,0:20:41.51,0:20:47.46,Default,,0000,0000,0000,,you will probably never use a 32 mask to configure an actual interface. However, there
Dialogue: 0,0:20:47.46,0:20:53.18,Default,,0000,0000,0000,,are some uses for a 32 mask. For example, when you want to create a static route not
Dialogue: 0,0:20:53.18,0:21:00.41,Default,,0000,0000,0000,,to a network, but to just one specific host, you can use a 32 mask to specify that exact host.
Dialogue: 0,0:21:00.41,0:21:05.71,Default,,0000,0000,0000,,Anyway, I'll talk about that later in the course. Just know that 32 masks are
Dialogue: 0,0:21:05.71,0:21:10.09,Default,,0000,0000,0000,,used at some points, but you don't have to worry about them for now.
Dialogue: 0,0:21:10.09,0:21:15.55,Default,,0000,0000,0000,,Here's a simple chart showing the dotted decimal subnet masks and their equivalent
Dialogue: 0,0:21:15.55,0:21:21.17,Default,,0000,0000,0000,,in CIDR notation. That's right, the way of writing a prefix with a slash followed
Dialogue: 0,0:21:21.17,0:21:29.78,Default,,0000,0000,0000,,by the prefix length, like 25, 26, etc., is called CIDR notation because it was introduced
Dialogue: 0,0:21:29.78,0:21:36.88,Default,,0000,0000,0000,,with the CIDR system. Previously, only the dotted decimal method was used. Note that
Dialogue: 0,0:21:36.88,0:21:41.63,Default,,0000,0000,0000,,I've shown you only how to subnet a class C network so far, but we will look at
Dialogue: 0,0:21:41.63,0:21:49.79,Default,,0000,0000,0000,,class B and class A networks as well, with prefix lengths like 17, 11, 9, etc.
Dialogue: 0,0:21:49.79,0:21:55.50,Default,,0000,0000,0000,,I spent a lot of time on just that one example, but I hope you can see the use of
Dialogue: 0,0:21:55.50,0:21:59.89,Default,,0000,0000,0000,,subnetting--dividing a larger network into smaller networks called subnets.
Dialogue: 0,0:21:59.89,0:22:07.13,Default,,0000,0000,0000,,Instead of using the whole 203.0.113.0/24 network for the point-to-point connection, we can
Dialogue: 0,0:22:07.13,0:22:14.07,Default,,0000,0000,0000,,use a 30 subnet and use only four addresses, or even better, use a 31 subnet and use only
Dialogue: 0,0:22:14.07,0:22:20.29,Default,,0000,0000,0000,,two addresses. I'll give one more example of subnetting before finishing up this video.
Dialogue: 0,0:22:20.29,0:22:23.87,Default,,0000,0000,0000,,In the next video, I'll give you some practice problems and walk you through them so you
Dialogue: 0,0:22:23.87,0:22:26.71,Default,,0000,0000,0000,,can get some hands-on practice with subnetting.
Dialogue: 0,0:22:26.71,0:22:33.62,Default,,0000,0000,0000,,So, here's a scenario: There are four networks connected to R1, with many hosts connected
Dialogue: 0,0:22:33.62,0:22:40.76,Default,,0000,0000,0000,,to each switch. There are 45 hosts per network. R1 needs an IP address in each network, so
Dialogue: 0,0:22:40.76,0:22:49.71,Default,,0000,0000,0000,,its address is included in that 45-host number. You have received the 192.168.0.14 network,
Dialogue: 0,0:22:49.71,0:22:54.08,Default,,0000,0000,0000,,and you must divide the network into four subnets that can accommodate the number of
Dialogue: 0,0:22:54.08,0:23:01.95,Default,,0000,0000,0000,,hosts required. First off, are there enough addresses in the 192.168.0.14 network in
Dialogue: 0,0:23:01.95,0:23:09.01,Default,,0000,0000,0000,,the first place? We need 45 hosts per network, including R1, but also remember that each
Dialogue: 0,0:23:09.01,0:23:15.80,Default,,0000,0000,0000,,network has a network and broadcast address, so that's plus two. So, we need 47 addresses per subnet.
Dialogue: 0,0:23:15.80,0:23:24.24,Default,,0000,0000,0000,,47 times 4 equals 188, so there's no problem in terms of the number of hosts.
Dialogue: 0,0:23:24.24,0:23:32.56,Default,,0000,0000,0000,,192.168.0.0/24 is a class C network, so there are 256 addresses. Therefore, we will be able to assign
Dialogue: 0,0:23:32.56,0:23:36.46,Default,,0000,0000,0000,,four subnets to accommodate all hosts, no problem.
Dialogue: 0,0:23:36.46,0:23:43.16,Default,,0000,0000,0000,,Okay, let's see how we can calculate the subnets we need to make. We need four equal-sized subnets
Dialogue: 0,0:23:43.16,0:23:50.45,Default,,0000,0000,0000,,with enough room for at least 45 hosts. Here, I've written out 192.168.0.10
Dialogue: 0,0:23:50.45,0:24:00.48,Default,,0000,0000,0000,,with a 30 mask, 255.255.255.252. I skipped 32 and 31 since these aren't point-to-point links.
Dialogue: 0,0:24:00.48,0:24:08.19,Default,,0000,0000,0000,,We can't use 31 and definitely can't use 32. Since there are two host bits,
Dialogue: 0,0:24:08.19,0:24:14.29,Default,,0000,0000,0000,,the formula to determine the number of usable addresses is
Dialogue: 0,0:24:14.29,0:24:20.97,Default,,0000,0000,0000,,2^2 - 2. 2^2 is 2 times 2, which is 4, so that means there are two usable addresses
Dialogue: 0,0:24:20.97,0:24:28.22,Default,,0000,0000,0000,,in a 30 network. Clearly, not enough room to accommodate the 45 hosts we have.
Dialogue: 0,0:24:28.22,0:24:36.33,Default,,0000,0000,0000,,How about if we use a 29 mask to make these subnets? Can we fit the 45 hosts we need? There are three host bits,
Dialogue: 0,0:24:36.33,0:24:43.71,Default,,0000,0000,0000,,so the formula is 2^3 - 2. 2^3 is 2 times 2 times
Dialogue: 0,0:24:43.71,0:24:51.91,Default,,0000,0000,0000,,2, which is 8. Therefore, there are six usable addresses, not enough for 45 hosts.
Dialogue: 0,0:24:51.91,0:25:00.25,Default,,0000,0000,0000,,How about if we use 28? There are four host bits, so the formula is 2^4 - 2.
Dialogue: 0,0:25:00.25,0:25:07.71,Default,,0000,0000,0000,,2^4 is 2 times 2 times 2 times 2, which is 16. So, that means there are
Dialogue: 0,0:25:07.71,0:25:13.41,Default,,0000,0000,0000,,14 usable addresses--once again, not enough for 45 hosts.
Dialogue: 0,0:25:13.41,0:25:23.40,Default,,0000,0000,0000,,How about 27? There are five host bits, so the formula is 2^5 - 2. And 2^5
Dialogue: 0,0:25:23.40,0:25:30.17,Default,,0000,0000,0000,,is 2 times 2 times 2 times 2 times 2, which equals 32. So that means
Dialogue: 0,0:25:30.17,0:25:35.37,Default,,0000,0000,0000,,30 usable addresses. Again, not enough for 45 hosts.
Dialogue: 0,0:25:35.37,0:25:43.74,Default,,0000,0000,0000,,How about a 26 subnet mask? There are now six host bits, so the formula is 2^6 - 2.
Dialogue: 0,0:25:43.74,0:25:51.48,Default,,0000,0000,0000,,2^6 is 2 times 2 times 2 times 2 times 2 times 2, which equals 64.
Dialogue: 0,0:25:51.48,0:25:59.12,Default,,0000,0000,0000,,That means there are 62 usable addresses. So, it looks like we've found our number. 27
Dialogue: 0,0:25:59.12,0:26:04.96,Default,,0000,0000,0000,,doesn't provide enough address space, but 26 provides more than we need, so we have to
Dialogue: 0,0:26:04.96,0:26:10.94,Default,,0000,0000,0000,,go with 26. Unfortunately, you can't always make subnets have exactly the number of addresses
Dialogue: 0,0:26:10.94,0:26:16.57,Default,,0000,0000,0000,,you want. There might be some unused address space. That's actually fine, since it's good
Dialogue: 0,0:26:16.57,0:26:20.50,Default,,0000,0000,0000,,to have some room for growth anyway.
Dialogue: 0,0:26:20.50,0:26:26.57,Default,,0000,0000,0000,,So, I think this video has gone on long enough. Instead of finishing this task in this video, I'll make
Dialogue: 0,0:26:26.57,0:26:36.66,Default,,0000,0000,0000,,it this week's quiz. The first subnet, subnet one, is 192.168.0.16. What are the remaining
Dialogue: 0,0:26:36.66,0:26:42.66,Default,,0000,0000,0000,,subnets? To help you out, here's a hint: Find the broadcast address of subnet one.
Dialogue: 0,0:26:42.66,0:26:50.12,Default,,0000,0000,0000,,The next address after that is the network address of subnet two. And then just repeat the process for subnets
Dialogue: 0,0:26:50.12,0:26:56.78,Default,,0000,0000,0000,,three and four. Post your answers in the comment section, and I'll also go over the answer in the next video.
Dialogue: 0,0:26:57.97,0:27:05.30,Default,,0000,0000,0000,,So, what did we cover in this video? We covered CIDR (Classless Inter-Domain Routing), which
Dialogue: 0,0:27:05.30,0:27:10.68,Default,,0000,0000,0000,,removes the rules of class A, B, and C networks and lets us be more flexible with network
Dialogue: 0,0:27:10.68,0:27:17.03,Default,,0000,0000,0000,,addressing, according to the size of the network. We also covered the process of subnetting,
Dialogue: 0,0:27:17.03,0:27:22.04,Default,,0000,0000,0000,,but mostly just the basics. Hopefully, you understand the purpose of subnetting and
Dialogue: 0,0:27:22.04,0:27:27.12,Default,,0000,0000,0000,,know a little bit about how to do it. I will clarify and expand upon many things in the
Dialogue: 0,0:27:27.12,0:27:33.96,Default,,0000,0000,0000,,next video, but also feel free to ask any questions you have in the comments section.
Dialogue: 0,0:27:33.96,0:27:38.76,Default,,0000,0000,0000,,For today's video, there won't be a practice lab; that will be after I've finished explaining everything about
Dialogue: 0,0:27:38.76,0:27:43.76,Default,,0000,0000,0000,,subnetting. There will be flashcards, however, to help you review some of the things learned
Dialogue: 0,0:27:43.76,0:27:48.12,Default,,0000,0000,0000,,in this video. You can download them from the link in the description.
Dialogue: 0,0:27:48.12,0:27:53.28,Default,,0000,0000,0000,,I've also recently enabled the membership feature for my channel. If you want to leave
Dialogue: 0,0:27:53.29,0:27:59.07,Default,,0000,0000,0000,,a monthly tip to support me, this is another great way to do so. Click "Join" here under
Dialogue: 0,0:27:59.07,0:28:00.65,Default,,0000,0000,0000,,the video to check it out.
Dialogue: 0,0:28:01.71,0:28:08.23,Default,,0000,0000,0000,,For those who become a JCNP (Jeremy Certified Network Professional) level supporter, I'll
Dialogue: 0,0:28:08.23,0:28:14.70,Default,,0000,0000,0000,,give you a shoutout at the end of my videos. So, first of all, thank you so much to Vance Simmons. I just
Dialogue: 0,0:28:14.70,0:28:19.08,Default,,0000,0000,0000,,enabled the membership feature and haven't said anything about it yet, and he became my first
Dialogue: 0,0:28:19.08,0:28:24.67,Default,,0000,0000,0000,,JCNP level supporter. Thank you so much for supporting the channel. I hope the videos are helping
Dialogue: 0,0:28:24.67,0:28:30.11,Default,,0000,0000,0000,,you out. And for my JCNA level supporters, thanks to you too.
Dialogue: 0,0:28:33.08,0:28:37.95,Default,,0000,0000,0000,,Thank you for watching. Please subscribe to the channel, like the video, leave a comment,
Dialogue: 0,0:28:37.95,0:28:43.03,Default,,0000,0000,0000,,and share the video with anyone else studying for the CCNA. If you want to leave a tip,
Dialogue: 0,0:28:43.03,0:28:48.65,Default,,0000,0000,0000,,check the links in the description. I'm also a Brave verified publisher and accept BAT
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