# CCNA Chapter 6

Discussion in 'Networking & Security' started by CheezusCrust, Oct 18, 2013.

1. ### CheezusCrustNew Member

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Hi everyone.

I'm currently studying for IT Support, and I've been nailing all of my tests so far with a 80%+ success rate, with the exception of Chapter 6 which is essentially subnetting & IPv6.

Despite putting extra effort & spending serveral evenings re-reading chapter 6, I only managed to get 60%. I've heard from my circle of hardware enthusiasts/IT Supporters that Chapter 6 is probably the hardest in the introductory course. I really want to nail this test properly.

Do you know any educational website teaching subnetting? It's mostly questions on:

• x.x.x.x /24, calculating the /24 properly, I just can't make any sense of it.
• Being able to tell how many hosts that are available based on something like 255.255.255.224

If this is the wrong subforum, please do tell me
Now, if you have any rule of thumbs with subnets, the /24 etc., I would greatly appreciate any information.
2. ### bencrutz

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IIRC, the /24 is a C class netmask.
subnetmask is 32bit, or in another way 4 segment of 8 bit.
e.g.
/24 is the same as netmask 255.255.255.0 or if we write it in whole 32bit binary it would be 11111111.11111111.11111111.00000000
so what is that 24 thing? well, it's how many 1s of the subnetmask (in binary ofc)

now, how much host can it be valid in /24 mask? well, it means theres 2^8 = 256 (8 is how many 0s int the netmask - a quick way to count if netmask /24 so the zeroes shoul be 32 - 24 = 8)
but remember that a 0 and 256 cannot be assigned to any host because 0 is for network address and 256 would be for broadcast address.

sorry if it's not clear enough, am no network expert
3. ### CheezusCrustNew Member

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That clarifies a buttload of stuff though, so thank you! So if there's a /24, it means the subnet mask is always 255.255.255.0?

What I don't get though is, if it's /25 (1 more) then everything seems to just shake up and the x.0 falls into something like x.248
4. ### FrickFishfaced Nincompoop

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Adding /24 is just a quick way of saying the subnet mask i 255.255.255.0. A /25 would mean the subnet mask is 255.255.255.128 (or x.10000000). This would give you two subnets and 126 hosts. A /26 (. 192 or .11000000) means four subnets and 62 hosts. And so on. The trick is converting binary to digital and vice versa. The /24 means the 24 first bits in the subnet mask are 1s (11111111.11111111.11111111.00000000 or 255.255.255.0). What we did in school was writing down all the subnets and hosts (not all of them, but plenty) in rows to get a feel for it. To me it made a lot more sense then, when you see the bits line up.

There are calculators for this stuff, but if you intend to get good at it do it yourself. And having binary/hexadecimal/decimal down is a great help as well (which is what I never got down properly).
CheezusCrust says thanks.
5. ### FlaxeNew Member

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Every time I'm doing a test which includes subnetting I'm drawing a table (picture below) which helps a lot. It's pretty easy to remember and it helps you doing almost any subnetting calculation in your head without having to write down lots of stuff.

The "Magic number" is also known as Increment. As you can see in this table, all subnetmasks written as /3, /11, /19 and /27 will end in x.224, depending on which octet. And the same goes for all masks obviously.
/3 = 224.0.0.0
/11 = 255.224.0.0
/19 = 255.255.224.0
/27 = 255.255.255.224

With a mask of /27, or 255.255.255.224, you have an increment of 32. This means every new subnet will start at 32. If we take the address 192.168.1.0/27 as example, you will have these IP-addresses written in this format: (net id, first usable-last usable, broadcast).
192.168.1.0, 192.168.1.1-192.168.1.30, 192.168.1.31
192.168.1.32, 192.168.1.33-192.168.1.62, 192.168.1.63
192.168.1.64, 192.168.1.65-192.168.1.94, 192.168.1.95
192.168.1.96, 192.168.1.97-192.168.1.126, 192.168.1.127
Etc...

As you can see, the number is only changing in the 4th octet. The 4th octet will change if you're using a subnetmask of /25 or higher. If you're using a subnetmask of /17-/24 you will change the third octet, as you also can see in the table. For example if you have the address 192.168.0.0/21
(net id, first usable-last usable, broadcast):
192.168.0.0, 192.168.0.1-192.168.7.254, 192.168.7.255
192.168.8.0, 192.168.8.1-192.168.15.254, 192.168.15.255
192.168.16.0, 192.168.16.1-192.168.31.254, 192.168.31.255
Etc...

As you can see, the increment (magic number) shown by the table tells you what subnet ID every new subnet will have and therefore you can easily calculate the IP-addresses inside of each subnet no matter what kind of IP-address or subnetmask you're using.
I used this website to learn and understand subnetting and I haven't failed a single time after I read it and understood everything. There's also a lot of subnetting questions together with answers and explanations you can do on that website also, which gives you the extra training needed to fully understand.
Note that there are 3 pages to read in this Subnetting Tutorial I linked to.

If you need help, just ask and I will try my best
I'm currently doing CCNA Chapter 3, LAN Switching and Wireless, and I'm going to do the Final Exam the upcoming week on Tuesday... maybe I can get some help from you if I need
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6. ### CheezusCrustNew Member

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Thanks for all the answers, appreciate the time you put into writing this!

I'll definitely have a look at it tonight, seeing as my teacher always makes things complex for no reason at all.

Which part of CCNA? Exploration? Discovery?
7. ### FlaxeNew Member

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Yeah, teachers usually make things more complex than they usually are, I know the feeling

CCNA Exploration.
8. ### CheezusCrustNew Member

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