Lecture 9: VPC Addressing (CIDR)
- 课程：[NEW] AWS Certified Advanced Networking Specialty 2026
- 视频时长：19:48
- 来源：Udemy Lecture 24111926
- Slide 数量：7
- 画面切换：约 00:28、03:38、06:26、08:20、13:57、17:40
扫描说明
按页面英文 transcript 的 290 个时间点逐一定位并检查视频画面，并对画面差异较大的时间点进行复核。相同幻灯片中的项目、二进制位和白板标注逐步出现均视为动画或连续推导，合并到对应小节中。
Transcript 来自 Udemy 页面自带的英文自动字幕，没有使用 ASR。下面保留页面原文，包括其中的语法或识别错误。
1. VPC CIDR
时间范围：00:00–00:28
总结 / Summary
中文
本节引出 VPC 的无类别域间路由（Classless Inter-Domain Routing, CIDR）地址规划。CIDR 是定义 VPC 网络边界和可用地址范围的基础。
English
This section introduces Classless Inter-Domain Routing (CIDR) as the addressing method used to define a VPC network boundary and its available address range.
Transcript
00:00 Hi.
00:00 So now let's get into the details of the first component that is Sidr.
00:06 This is how we address our VPC and believe me, most of the people find it most confusing thing to understand.
00:13 But after this lecture, I'm sure you will feel much confident about the VPC.
00:19 Cidrs.
00:21 So very first thing before we understand VPC Sidr is understanding the IP addresses.
2. IPv4 and IPv6 Addresses
时间范围：00:28–03:38
总结 / Summary
中文
互联网协议地址（Internet Protocol address, IP address）用于标识网络中的主机。IPv4 地址（IPv4 address）为 32 位，通常由四个八位组（octets）以点分十进制表示；IPv6 地址（IPv6 address）为 128 位，拥有远大于 IPv4 的地址空间。课程通过二进制（binary）转换说明每个 IPv4 八位组如何表示十进制数值。
English
An Internet Protocol (IP) address identifies a host on a network. IPv4 addresses contain 32 bits and are commonly written as four decimal octets, whereas IPv6 addresses contain 128 bits and provide a vastly larger address space. The lecture also reviews how each IPv4 octet maps between decimal and binary representation.
Transcript
00:28 So I'm sure you understand the IP address scheme.
00:31 So there are two types of the IP addresses IPv4 and IPv6 addresses.
00:37 Now so far IPv4 addresses were sufficient.
00:40 But with the advent of Internet of Things, IoT and so many other applications and massive digital transformation,
00:51 we know that we are exhausting the IPv4 address range.
00:54 And that's where now IPV six range has been introduced, which is much, much larger than IPV four address
01:03 range.
01:03 And for foreseeable future, we can't exhaust IPV six addresses because they are 128 bit in length as
01:11 opposed to IPV four addresses, which are just 32 bit in length.
01:16 Okay, let's start with the IP.
01:18 Why you need IP.
01:19 So IP is Internet protocol, and every machine on the internet should have the IP address so that we
01:27 can reach to it.
01:28 Right.
01:29 So IP address is an identity of the host in the network.
01:33 Now as I said there are two types of the IP addresses IPV four which is 32 bit in size.
01:41 And then IPV six address is 128 bit in size.
01:45 Now every bit represents the power of two.
01:49 That means for IPv4 addresses we have two raised to 32 those many IP addresses available.
01:56 And for IPV six it is two raised to 128.
02:00 So in this lecture we will mainly focus on IPv4 addresses because it's easier to demonstrate.
02:06 But same logic applies for IPv6 addresses as well.
02:11 So just a quick refresher on IPV four addresses how they are represented.
02:16 So you know that there are 32 bit.
02:19 So in that there are four octets.
02:22 So here you see 12.34.56.67.
02:27 So there are four eight bits.
02:29 And every eight bit octet has been separated by a dot.
02:34 Right.
02:34 So it ultimately represents the decimal value of that octet.
02:39 So for example if you have an IP address 192.168.56.212.
02:47 In that case, if you want to represent that as a 32 bit, then you would have to convert it into the
02:54 binary as an each octet.
02:56 And then you can see the binary value of that.
02:59 So 192 can be represented in a binary.
03:03 As you know this calculations you have to turn on the bits which makes 192.
03:09 So bit number seven and bit number six.
03:12 And if you look at the binary then it should look something like this.
03:17 So that's 192.
03:19 Similarly you can have 168.
03:23 56 and two and two.
03:26 So this is just a binary representation of the IPv4 address.
03:31 So as I said, maybe in the college days or before that you might have learned different addressing
3. CIDR – Classless Inter-Domain Routing
时间范围：03:38–06:26
总结 / Summary
中文
无类别域间路由（Classless Inter-Domain Routing, CIDR）取代了传统 A、B、C 类地址方案。CIDR 由基地址（base address）和前缀长度（prefix length）组成，例如 /16 表示前 16 位固定为网络位（network bits），其余 16 位作为主机位（host bits）。前缀越长，可用于主机的位越少，网络内可用的总地址数也越小。
English
Classless Inter-Domain Routing (CIDR) replaces the older Class A, B, and C addressing model. A CIDR consists of a base address and a prefix length. For example, /16 fixes the first 16 bits as network bits and leaves the remaining 16 as host bits. A longer prefix leaves fewer host bits and therefore defines a smaller address range.
Transcript
03:38 scheme.
03:38 And that was address of class A, class B or class C.
03:43 So if you talk about class A address first octet used to be reserved for network addressing.
03:49 And rest of the three octets were reserved for the host addressing in that network.
03:55 Similarly, different combinations for class B and class C type of the addresses.
04:00 But now this scheme has been replaced with the Sidr.
04:04 And in case of Sidr, you have the base address and you have something called prefix.
04:11 So here 192.168.0.0 is a base address and slash 16 is the Sidr prefix.
04:22 But what is this slash 16 and what does it mean.
04:25 So let's try to decode that.
04:28 So this is the Cidr notation.
04:30 And now if you represent that as an binary IP address then we just learned that you can convert every
04:38 octet into binary.
04:39 And this is how it will look.
04:41 So there are 32 bits.
04:43 And we are saying the Sidr prefix is 16.
04:47 So what.
04:48 It means that 16 bit we are locating to the network address and rest of the 16 bit out of 32 we are
04:58 allocating for the host addressing inside that network.
05:02 So this 16 prefix tells us how many bits we are allocating for the network.
05:09 So network address is this.
05:11 And within that network you have this many hosts.
05:15 So if you create a VPC then this is a network address.
05:20 And individual host can receive any of the IP address from this range.
05:25 What that means is for the VPC, 192.168 is a fixed address.
05:33 And then as you go on creating the host inside the VPC, that means EC2 machine.
05:39 Then every machine will receive the IP address from the range 0 to 2, 55, and 0 to 255.
05:48 And shortly we will see with the whiteboarding how this addresses can be allocated to the different
05:54 hosts and how many IP addresses you can have inside a VPC when you create that VPC with this kind of
06:01 Sidr range.
06:02 But from this slide, I want you to understand that whatever prefix you mention, those many bits are
06:09 allocated to the network and you can't change that.
06:12 But rest of the bits which are available to you can be used for individual IP addresses inside that
06:20 network, and those can be assigned to the host.
4. Whiteboarding the /16 Address Range
时间范围：06:26–08:20
总结 / Summary
中文
课程以 10.10.0.0/16 为例开始白板推导（whiteboarding）。/16 固定前两个八位组，后两个八位组可以变化，因此地址从 10.10.0.0 延伸到 10.10.255.255。地址数量可用公式 2^(32−前缀长度) 计算，即 2^16 = 65,536。
English
Using 10.10.0.0/16, the whiteboard walkthrough shows that the first two octets remain fixed while the last two can vary. The range therefore extends from 10.10.0.0 to 10.10.255.255. Its size is calculated as 2^(32 − prefix length), or 2^16 = 65,536 addresses.
Transcript
06:25 Let's say an example.
06:27 I have a VPC and now in this VPC I give some address.
06:33 So this is why VPC I give it address ten dot ten .0.0 slash 16.
06:39 Now as I said, when you create a VPC with this address range out of this network address, first 16
06:47 bits are given to the network.
06:49 This 16, which is the prefix that means for this VPC.
06:52 I cannot change this for any of the host, which I'm going to launch inside this VPC.
06:57 What I can change is only this two octets, right?
07:01 So if you see the first address in this VPC will be ten .10.0.0.
07:08 Now I can change only this part.
07:11 So the next address will be ten .10.0.10.20.3.
07:17 Likewise, maybe up to 255, because we are talking about IPV four addresses and this is one octet,
07:25 right?
07:25 Eight bits.
07:26 Now my addresses does not stop here.
07:30 Once I'm done, I will again write ten dot ten dot now 255 is done.
07:36 So I need to increment this bit now, so I'll make it 1.0 again.
07:41 I continue 1.11.21.3.
07:44 Likewise my.
07:47 Address here would be ten .10.1.255.
07:52 Does my address end here?
07:55 Not really.
07:56 Right.
07:56 I can now go on incrementing this bit again.
08:00 The next address would be ten dot ten .2.02.12.2.
08:07 Likewise 2.2 55.
08:11 So that means I can go on increasing this bit now after every 256 addresses.
08:16 So how many addresses I have in this network.
5. Dividing a VPC into /24 Subnets
时间范围：08:20–13:57
总结 / Summary
中文
VPC 的 CIDR 块（CIDR block）必须先划分为子网（Subnets）才能放置 EC2 实例。将 10.10.0.0/16 划分为 /24 子网时，每个子网保留 8 个主机位，因此包含 256 个地址；示例依次形成 10.10.0.0/24、10.10.1.0/24、10.10.2.0/24 等互不重叠的子网。VPC 最多可由 256 个这种大小的地址块覆盖。
English
A VPC CIDR block is divided into subnets before EC2 instances can be placed in it. Splitting 10.10.0.0/16 into /24 subnets leaves eight host bits in each subnet, producing 256 addresses per subnet. The resulting non-overlapping ranges include 10.10.0.0/24, 10.10.1.0/24, 10.10.2.0/24, and so on, with 256 such blocks covering the original /16.
Transcript
08:20 So I have 256 in every block.
08:23 Likewise I have 256 blocks.
08:26 So 256 into 256 which is 65536.
08:32 Now you can make the same calculations here itself.
08:35 When you create a VPC with this Sidr 16 bit you are giving to the network, that means rest how many
08:44 bits you have for the host addressing 32 -16, which is 16 address bits you have for the host.
08:51 So total number of hosts you can have in this network is two raised to 16, which is again 65536.
09:00 It's as simple as that.
09:02 So the formula is whenever you see some Sidr for example.
09:06 Now let's take another example ten dot 100 .0.0 slash 24.
09:13 If you see this sidr and someone asks you how many IP addresses would be there in this Sidr.
09:19 So just look at this number 24.
09:22 That means this many bits are reserved for my network.
09:25 I only have 32 -24 equal to eight bits available for my host addressing.
09:31 So total number of host I can have is two raised to eight.
09:36 That is 256.
09:37 That means as big this Sidr is, the smaller your network is, because those many bits you are reserving
09:45 for the network and rest of the bits are available for host addressing.
09:50 So I hope this is somewhat clear.
09:53 Now let's dig it further.
09:54 Again.
09:55 Let's see we I have a VPC right.
09:58 And.
10:01 Similar address ten .10.0.0 slash 16.
10:06 But as I said, when you create a VPC, you can't use it as it is.
10:10 You need to first create subnets if you want to launch a EC2 instance within that VPC.
10:16 Now what I decide is I need a subnet, and every subnet that is a smaller network within VPC should
10:23 be of size 256 each.
10:26 So excuse my drawing here, but my first subnet should have 256 addresses.
10:33 So let me just write down those addresses.
10:36 So first address of course the range from which the VPC starts.
10:41 Then ten .0.012 up to 255.
10:47 Now this is my first subnet.
10:48 My second subnet would be ten dot ten .1.0.
10:53 So this is exactly same what we did earlier.
10:56 But just we are now calling it as one different subnet than the other.
11:01 Likewise it would have 255 I can just go on.
11:06 Adding more subnets.
11:07 So the next subnet would be ten .1.2.0.
11:11 And then it would have up to 255 256 total IP addresses.
11:16 Right now, if I want to write a Cidr notation for this subnet, you can very well write the first.
11:23 Always write the first starting address there that is ten .1.0.0.
11:29 And then understand how many IP addresses are there 256 256 means two raised to eight.
11:36 So for host we have eight eight bits.
11:38 That means we are giving 24 bits to the network.
11:42 So.
11:43 My subnet Cidr will be ten .0.0.0.
11:47 That is starting address slash 24.
11:50 And you can, uh, very well uh, cross verify this.
11:53 If you look at this whole IP address range, this three bits are not changing.
11:58 So the three octets are not changing for this subnets.
12:00 They are fixed.
12:01 And that's where it's 24.
12:03 Only this part is changing.
12:05 That is eight bits.
12:06 And ultimately you can make the same calculation 32 -24 given to the network eight bits are for host
12:13 addressing.
12:13 So total number of host equal to two raised to eight equal to 256.
12:19 Right now the same for this subnet.
12:21 This subnet starts at this address.
12:23 So we need to write a Cidr ten .10.1.0.
12:28 Right.
12:28 Which is my starting address.
12:30 Now again this subnet is of size 256.
12:32 So you need to write a prefix of 24 again.
12:37 So now this is my second subnet.
12:39 Similarly my third subnet would be ten .1.2.0 slash 24.
12:45 Likewise I have many such subnets and my last subnet would be ten dot ten dot here 250 5.0 slash 24.
12:58 Now again I have 256 size of each subnet and 256 such subnets.
13:05 So again the total size is 256 into 256 which is 6553 sorry three six.
13:13 Right.
13:14 And the same calculation you can do that from the VPC out of 32 bits I am giving 16 to the network.
13:22 So I have 16 for my host addressing within that VPC.
13:25 So total addresses 655 uh three six.
13:30 Right.
13:30 Exactly.
13:31 Same.
13:31 So it really depends on us how we divide this VPC into form of subnets.
13:36 It's not necessary that you always create subnet of size 24.
13:39 You can very well create a bigger subnet with say 23 or 20 2 or 21 or even the 16.
13:47 So you can very well just create one VPC, right, which has an address range, say ten dot ten .0.0
6. Single-Subnet and /28 Examples
时间范围：13:57–17:40
总结 / Summary
中文
子网大小由设计需求决定，并不必须是 /24。一个子网可以与整个 VPC 使用相同的 /16 范围，也可以使用 AWS VPC 支持的最小 IPv4 子网前缀 /28。/28 保留 4 个主机位，每个子网包含 16 个地址，因此地址边界按 16 递增，例如 10.10.0.0–10.10.0.15、10.10.0.16–10.10.0.31。
English
Subnet size is a design choice and is not limited to /24. A single subnet can use the full /16 range of its VPC, or smaller subnets can use the minimum IPv4 subnet prefix supported in an AWS VPC, /28. A /28 leaves four host bits and contains 16 addresses, so its boundaries increase in blocks of 16, such as 10.10.0.0–10.10.0.15 and 10.10.0.16–10.10.0.31.
Transcript
13:56 slash 16 and I.
13:58 And then you can say I don't want to create multiple subnet.
14:01 I'm sure that I just need one subnet in in this VPC.
14:04 Uh, I can't think of the use case, but just let's imagine and then I can just create one subnet.
14:11 So right insert this say subnet A.
14:13 And I can have the same IP address range as my VPC now which means this subnet starts from this address.
14:22 And this subnet ends at this address.
14:26 And there is no other subnet because we just created one subnet right now.
14:31 That's interesting.
14:32 Right.
14:32 That means it's up to you how many subnets you want as long as you do not reach the VPC limit.
14:39 Now let's take one more example.
14:41 And with that I'm sure, uh, the Cidr will be much clearer.
14:45 Uh, so again the same example we have VPC with say ten .10.0. 16.
14:51 Now this time I want to create even the smaller subnets.
14:54 I don't want subnet of size 256 as I want subnet as small as I can because then I want to create many,
15:01 many subnets, not just 256.
15:03 Right.
15:04 So the smallest subnet you can create inside VPC is with slash 28 prefix.
15:09 Right.
15:10 And why.
15:10 So I'll talk about that.
15:11 But let's understand if you are creating subnet of size now uh with prefix slash 28 how many IP addresses
15:19 you would have again in the subnet.
15:22 Uh 32 -28 which is four bits.
15:24 So two raise to four two raise to four equal to 16 hosts or IP addresses you can have in this subnet.
15:32 That means my first uh subnet.
15:38 Sidr would be this.
15:40 This is slash, and the first address would be this .1.2.
15:45 Now, how long it will go?
15:48 Look at the number of, uh, IPS.
15:50 The subnet would have only 16 IP addresses and we started at zero.
15:55 That means the last IP address in this subnet would be this.
15:58 So it will start at ten .10.0.0.
16:03 And it will end at ten dot ten .0. 15.
16:06 Now that's my subnet right.
16:08 Because I'm creating a smaller subnet.
16:10 Now what would be the next subnet again ten dot ten dot after this.
16:15 Right.
16:16 Zero dot 16.
16:18 Now next subnet would start here again prefix slash 28.
16:22 It will go on like this 1718.
16:26 And of course again it would have 16 IP addresses total.
16:29 So ten dot ten .0. 31.
16:33 Now this would be the last IP address in this subnet.
16:36 Right.
16:37 So now my subnet ends here.
16:39 Likewise now you're creating a smaller subnet right.
16:42 And then how many subnets you can create.
16:45 You know total IP address in this VPC is this.
16:48 And every subnet is of size 16.
16:50 So whatever number it comes those many subnets you can create uh and you can go on as long as you hit
16:57 the last uh address in this VPC.
16:59 That is ten dot ten .255.255.
17:05 Right.
17:06 So with that, I hope if given any sidr, you can very well make out, um, what how many IP addresses
17:15 will be there in that network?
17:17 And when you create a network with slash 28, you have to raise to four equal to 16 IP addresses.
17:23 If it's say 27, it's two raised to five, that is 32.
17:27 Similarly, if it's 26 then two raised to six equal to 64.
17:33 So all this calculation you can now easily make.
17:37 So uh let's try to solve this.
7. Understanding CIDRs – Exercise
时间范围：17:40–19:48
总结 / Summary
中文
练习使用 2^(32−前缀长度) 快速计算 CIDR 地址数量：/24 有 256 个地址，/16 有 65,536 个地址，/32 只精确表示一个 IPv4 地址。0.0.0.0/0 的前缀长度为 0，会匹配所有 IPv4 地址，常见于需要面向整个互联网开放的防火墙规则（firewall rules）。
English
The exercise applies 2^(32 − prefix length) to common CIDRs: /24 contains 256 addresses, /16 contains 65,536, and /32 identifies exactly one IPv4 address. The prefix 0.0.0.0/0 matches every IPv4 address and commonly appears in firewall rules intended to allow access from the entire Internet.
Transcript
17:39 If you have the IP address range 19216800 slash 24 again 24.
17:45 That means 32 minus right 32 -24 equal to eight bits for the host.
17:52 So two raised to eight equal to 256 IP addresses I would have.
17:56 It will start with ..0.
17:58 It will add it dot 255.
18:01 This is what we just understood.
18:03 Slash 16.
18:04 Of course you would have 65536.
18:07 That is 16 -16.
18:08 That is 16 bits available for the host addresses two raised to 16.
18:12 Is this number.
18:13 Now this is interesting.
18:15 134 5678 123 slash 32 now make the same calculation 32 -32 equal to zero.
18:24 So two raised to zero equal to one.
18:26 That means in this network I can have only one IP address.
18:29 And it is itself right.
18:31 This the same IP address which is given here.
18:34 So that's that's what the slash 32 means.
18:38 So if you want to specify a single IP address using the Sidr notation it would have slash 32.
18:44 Similarly 0000/0 is interesting.
18:47 This means all IP address.
18:49 You are not reserving anything for the network right.
18:52 So anything on the internet can be, uh, really matched with this IP address.
18:57 So typically you will see in the security groups whenever you configure um, uh, firewall rules.
19:04 And if you want to open that port for the whole world, like for example port 80, that is Http, you
19:09 want to open for whole world, you will open the source IP as 0000.
19:16 That means any IP it can match.
19:18 Right.
19:18 So remember this, uh, and if you want to make your calculation and make your life easy, you can go
19:23 ahead and check this website.
19:25 Uh, and you can just enter your Cidr range and it will exactly show you which IP addresses, uh, will
19:31 be there in that, uh, given Sidr.
19:34 But I hope with this explanation you can make out, uh, on your own.
19:39 So that's it about the VPC Sidr.
19:41 And now let's look into the other components of the VPC.
19:45 Thanks for watching and stay tuned.
整讲总结 / Overall Summary
中文
无类别域间路由（Classless Inter-Domain Routing, CIDR）用“基地址 + 前缀长度”同时描述网络边界和规模。IPv4 网络包含 2^(32−前缀长度) 个地址；前缀越长，网络越小。VPC CIDR 可以按需求划分为多个不重叠的子网 CIDR，理解网络位（network bits）、主机位（host bits）、块大小和边界递增规律，是规划 VPC 地址空间的基础。
English
Classless Inter-Domain Routing (CIDR) describes both a network boundary and its size through a base address and prefix length. An IPv4 CIDR contains 2^(32 − prefix length) addresses, so a longer prefix defines a smaller network. A VPC CIDR can be divided into multiple non-overlapping subnet CIDRs; understanding network bits, host bits, block size, and boundary increments is fundamental to VPC address planning.
返回顶部 ↑