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Okay, so now let's talk about CIDR or CIDRs

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or Classless Inter-Domain Routing,

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which I will call CIDR for the rest of this course.

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So, it's a method for allocating IP addresses

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and we've seen them before in this course,

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because when we look at security group rules for example

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and networking in AWS, we saw something like this.

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So, these are my security group rules

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and if we look at the source column,

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here we remember we had an IP address / and then a number

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and this was our first touch with CIDRs.

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So, CIDRs help define IP ranges

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that we've seen some very, very simple one.

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So, when we have an IP address /32,

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this actually represents only one IP.

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And when we saw 0.0.0.0/0

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then this is represented all IPs,

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but there are a lot of different possibilities.

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For example,

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if we define 192.168.0.0/26

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then this represents a range of 64 IP addresses

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as you see in this slide.

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So, how does CIDR work?

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Well, CIDR has two components.

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There's the base IP and this is an IP that is contained

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in the range.

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Usually it is the beginning of the range,

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but it could be that it's contained in the range.

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And then, so this is example, so 10.0.0.0 or 192.168.0.0.

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And then the other thing is the subnet mask

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and the subnet mask will define how many bits

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can change in the IP.

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So, we have the /0, /24 all the way to /32

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and this can take two different forms.

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So, if we have /8 this is equivalent

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to subnet mask of 255.0.0.0

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and /16 is 255.255.0.0

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and so on, okay.

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But the most common form we see in this course and in AWS

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is the / form, okay?

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So, for the rest of this course

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we'll be using /8, /16 and so on.

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Now, let's have a detailed look at subnet masks.

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So, this is how we can define

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which values can change from the base IP.

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So, if we have an IP address /32,

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this allows for one IP, 2 to the power of 0

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and this is the IP that we have on the left hand side.

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Now, if we have an IP /31,

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then we allow for two different IPs.

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And so we'll allow for 192.168.0.0 and then .0.1.

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If we have the same IP /30,

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now this goes exponentially as you can see,

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we allow for 4 IPs.

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So, we'll have a bigger range from .0 to .3,

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then /29 allows for 8 IPs so .0 to .7.

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/28 allow for 16 IPs, so .0 to .15,

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then we can look at the table, okay.

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So, it goes from 4, 8, 16, 32, 64, 128, 256 IPs to change.

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So, the /24 is 256 IPs, which is 2 to the power of 8

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and this is from .0 to .255.

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Then if we go one step deeper,

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so we go to /16, so I skipped some in the middle.

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Then we allow for 2 to the power of 16 IP to change,

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so 65,536.

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So, as we can see the last two parts of the IPs have changed

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and then /0 allows for all IPs,

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so from the very beginning to the very end.

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So, how do we know?

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As a quick memo we know an IP is made of 4 Octets,

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so the first part, the second part,

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the third part and the fourth part.

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And /32 means no octets can change,

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/24 means the last octet can change,

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/16 means the last 2 octets can change

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and take different values.

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/8 is the last three octets can change

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and /0 is all octets can change.

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So, hopefully they started making a little bit more sense

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into how CIDRs work.

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So, now here comes a little exercise,

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feel free to pause the video if you need to answer.

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So, what is 192.168.0.0/24?

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So, /24 so that means the last octet can change

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and so therefore we have 256 IPs,

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because the last number it can go from the number 0

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to the number 255 included.

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Now, what is 192.168.0.0/16?

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Well, this is /16,

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so the last two octets can change,

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so we have a 65,536 IPs.

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And what is 134.56.78.123/32?

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Well, /32 so it's only one IP, so just one IP.

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And then 0.0.0.0/0 we know this one,

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it is all IPv4 space, okay?

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When in doubt, there is this little website

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that I can show you that is very, very handy.

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So, on this website we have two options,

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we can have a CIDR to an IP range

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or an IP range to a CIDR, so let's try both.

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So, if you have 10.0.0.0/ let's say a 16,

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we calculate it as you can see we see the first IP

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and the last IP in here.

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How many IPs are including to the range and so on?

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Okay.

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If we do /17, which is a little bit less obvious.

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In this one, we have 32,000 or so IPs.

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We can have a look at the first bit so

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this is the first IP and this is the last IP,

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so it's quite handy and again you can play.

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And reversely if you wanted to for example,

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take an IP range, so this IP range and figure out

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what CIDR this corresponds to, then click on calculate

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and this will give us a result of 10.0.0.0/17.

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So it's quite handy, if you start doing some networking

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to use a website like this to help you out.

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So, why are we talking about all these things?

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Well, because we want to discuss public versus private IP

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in the IPv4 space.

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So, there is the Internet Assigned Numbers Authority

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the IANA that established certain blocks of IPv4 addresses

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that will be used only for private LAN networks,

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so local network or public internet addresses.

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So, the private IPs can only allow for certain values.

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So we have this range for 10.0.0.0/8

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which represents, you know a huge amount of IPs.

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So, this is a private IP range you can use

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and this is your least something you use in big networks,

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because it allows for lot of different values.

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Then we have the 172.16.0.0/12,

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which is another sets of private IP addresses.

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And when you create an account

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it will provide you with a default VPC

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and it turns out the default VPC

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will be included in that networking space.

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And then finally,

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another one you may have seen before is 192.168.0.0/16,

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which represents your home network.

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So, this is very common when you have an internet router

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to have your own devices,

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have an IP that starts with 192, okay?

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So, all the other IP addresses out there in the world

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are going to be on the internet

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and they're going to be public IP addresses.

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So, that's it for these basics of IPv4s and CIDRs,

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I hope you liked it and I will see you in the next lecture.