# Classless Inter-Domain Routing (CIDR) to Create Subnets Part 2

Video Activity
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Time
8 hours 20 minutes
Difficulty
Beginner
CEU/CPE
8
Video Transcription
00:05
it is entirely possible you're someone who doesn't enjoy binary. That's okay.
00:09
In order to submit, we have to have a degree of understanding. If you're not going to be doing, submit mass conversion or binary and decimal on the exam, you'll need to know this chart.
00:20
If we steal one bit, that significant Octa you can see it will be one in seven zeros in the decimal place. We'll see 128 with two as the number of subnets created
00:31
when we moved to stealing. Two bits are decimal is 192 and we create four sub nets.
00:37
If, for example, we need 30 networks created if it's originally a class A address. The first octet is 255 and the second oct it from which will steal is called the significant Octet. If I need 30 networks, the sub net mask I use would be to 55.248 point 0.0, and that would give me support for 32 subnets.
01:02
No, and memorize this chart. You can probably see the pattern here, so it isn't brutal memorization for bits stolen. You don't necessarily have to memorize up to nine. It really depends on how many bits you need to steal, which is driven by how many subnets you need to create
01:18
with a significant octet each time we're stealing an additional bit,
01:22
looking at the decimal places to go from 128 to 192 is 64
01:29
going from 192 to 224 is 32
01:34
and from 224 to 240 is 16, 240 to 248 is eight.
01:41
So what I'm trying to point out here is a pattern in the decimal usage.
01:44
Just a tip to help you remember your subnets. Created are also a pattern 248 16 doubled Each time
01:52
I recommend being able to create this sub net mask chart because it will be critical to being able to submit, which you'll see a lot of on the test.
02:00
Somebody will appear on the security plus exam, too.
02:05
Let's say I get a question that essentially says, Start with a class A address, and it needs to be into subject 22 additional subnets
02:13
on the left. We have our instructions. The first thing I look for is the class of the original network, and that's a 10 that tells me it's a class a address just going with the defaults. If that's the case, the default mask for Class A is 255.0 point 0.0. I'll be stealing bits from the second octet called the Significant Opted.
02:35
Now I use the chart
02:37
to create 22 subnets. I need to go up to 32 because 16 is not enough. That has any ceiling five bits and creating 32 subnets. The mask will need to use is 255.248 point 0.0.
02:53
How about your time? What I'd like you to do is answer the handful questions here by pausing the video to figure these out,
03:00
take a look at certain addresses in classical sub net masks, then assume you want to submit them.
03:06
Use the chart that I've treated for you on the previous slides to figure this out.
03:09
That said, I do want you to pause and work through this on your own now because it will be included on the exam.
03:16
Mhm.
03:17
I hope you did pause the video and had a chance to look through the questions. Here we have the answers for question two. What sub net mask would you use for a Class A subject needing 68 networks or sub networks?
03:30
If we look at stealing seven bits, which is a lot, it's necessary because six bits is not enough
03:37
to support 68 subnets. I've got to steal seven bits. Which leaves me with 255.254 point 0.0.
03:45
Next question.
03:46
What sub net mask would you create on the 17.16 point X Point X Network to create five sub nets?
03:53
Create five. I don't need to see a lot of bits. Stealing three bits gives us 255.255 point 2 to 4.0.
04:03
Notice that because we're working with a Class B, the first two octaves are already set for me.
04:10
The significant octave becomes the third year.
04:13
What sub net mask would you use to create 12 subnets on a Class C network
04:16
with a Class C network? We're running out of room because we've only got four. Oct it's and the first three are already set
04:24
two to the fourth. Power creates 16 subnets, which gives us 255.255 point 255.24 Oh,
04:32
what some that mask would you use to create 300 subnets on a classy network? This is tricky.
04:40
If I have a Class C network, the first octet is set.
04:43
If I only steal eight bits, that will only support 256 hosts. So I need to double that. I need to steal a bit of the third octet, giving me support for 512 subnets covering the 300 that I need.
04:58
Please make sure you're comfortable with these and review the previous section is needed because we'll build on implementing these I P addresses next
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