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### Video Description

The topic of subnetting can be tedious and challenging for many. This section is excellent at explaining how subnetting works, how IP addresses are broken down, and the purpose of subnetting in security and network administration and what network components are represented. You'll oversee a demonstration of how a subnet is configure, in minute, where the citation is broken down in mathematical detail, and you learn how to read a subnet mask address and what each segment of the address means/correlates to. You'll also learn how that is uses/applies to the IPv4 address nomenclature. [toggle_content title="**Transcript**"] Hello, my name is John Oyeleke; subject matter expert for the security plus exam that is the CompTIA security plus SY0401. Today I will be talking about subnetting. This is from section 1.3 of the CompTIA syllabus. Subnetting is very interesting topic. Some people like to shy away from it. It could be pretty tedious but it's very easy if you understand the steps. The principle of subnetting is that you take one IP address and you break it down using a CIDR notation to a group of several addresses. Let's take IP address 192.168.1.10. We require to subnet this to /27. There are a couple of things we need to write down. We have our block size over here. We say 1, 2, 3, 4, 5, 6, 7, 8. These are 128, 64, 32, 16, 8, 4, 2 and 1. For some mathematics later on we will need so we take 1, 2, 3, 4, 5, 6, 7 so 21 will give us 2, 4, 8, 16, 32, 64. We could have this run all the way to 1015 but this is about sufficient for what we need right now. Using CIDR notation rules we know has to subnet to 27. This tells us we are using a subnet mask of 255.255.255.0. Basically the subnet mask tells us what portion of the IP address is the network address and what portion is the host address so every portion with the 255 is a network address and the portion with 0 is the host address. Looking at this address, we know we've used up 24 bits, 8 times3, 24 but we are going to 27 so what do we do? We need to do this; we say this can be 1, 2, 3, 4, 5, 6, 7, 8 representing the 8 bits we have in an octet so we used up 24, we are going to 27. We need to borrow 3 bits. 1, 2, 3. Then we can calculate the first, the number of subnet using this formula. We say 2 to the end where end is the number of bits borrowed. This gives us 2 to the 3. 2 to the 3 will give us 8 subnets using that so our network ID given this will be 192.168.1.0. This tells us that we are going to have 8 subnets so that means we are going to add this 8 times so we say 192.168.1, 192.168.1...1, 2, 3, 4 1, 2, 3, 4, 5, 6. We need 2 more so what will be the last value? We look back here. What is the value of the last bit borrowed? 1, 2, 3. The value there is 32 so that means this will increase by 32 each time. 32 there would be 0, 5, 6. Got that right. And 32 here will be...so that's it. We have 1, 2, 3, 4, 5, 6, 7, 8 29 subnets. We then look back here. How many bits do we have left? We have 1, 2, 3, 4, 5. We bring this into another formula. 2 to the end minus 2 where end here is the number of bits we have left. This gives us 2 to the 5 Minus 2. Look here 2 to the 5 is 32, minus 2 this gives us 30 pieces per subnet. Let's try and prove that. This calculation tells us we can get 30 computers per subnet so let's show that. What we do is we create this chart. Nice and simple chart to prove the number the computers you have per subnet. This is your network ID. This is your first valid, and here you have your broadcast. We simply add in 1 to the last octet over here to get the first valid. Here will be 192.168.1.1. You add 1 to the last value.33.65.97.129.161.193.225. The next one we find is the broadcast. To get the broadcast we subtract 1 From the next level down so this would be 192.168.1.31 so we have 31.63. 95.127.159.191.224 To get the next one you add 32 to this that would be 256 so we have 255. How do we get the last valid? We subtract 1 from the broadcast. That that way this would be 192.168.1.30 and you go all the way 62.94. 126.158.190.223. That is for that, that is for that, that is for that. Oh I think I've gone too high. Come back down a little bit. We have 158.190.223 and 254. This tells us we are going to have 30 computers per subnet. There is the proof; 1 the first valid, 30 the last valid. Between 1 and 30 you have 30 computers. These are your first addresses, your last addresses. In this formula we have this 2 we cannot use 2 that is we cannot use broadcast we cannot use the network ID. So that's it. We have been able to show the chart to prove the number of computers. We have 8 subnets and this how we subnet IP addresses. Thank you. [/toggle_content]

### Course Modules

### CompTIA Security+

#### Instructor Background

#### Module 1

#### Module 2

#### Module 3

#### Module 4

#### Module 5

#### Module 6

#### Exam Module

### Instructed By

CISSP CISM CISA CHFI CSXF CEH, Cyber Security Specialist & Trainer

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