CIDR Notation ?

kriscamaro68

So I was wondering how much CIDR notation you have to know for the new ccent/ccna exam.

Also I am learning this and I can convert ip addresses from decimal to binary and vise versa but I have a problem with firguring out the network id and the broadcast id in a quick manner. If this was on the test I would be on the question for awhile cause i suck at math. Do any of you have suggestions or ways that you learned it that could help me out.

Also are the only differences between subnetting and CIDR the fact that one is based on classful ip addressing and CIDR is for the most part not. I know that you use class c ip range to determine how many ip addresses are in what ever / your using (example: /20= 4094 or 256x16-2) but that is the only thing I see it using from the classful method. From what I can tell in subnetting you are stuck with a default subnet mask for a class c where with CIDR you have all the different /x and so on that you can use which lets you use a broad range of subnets depending on your situation. Any help in clearing this up would be appreciated. Thanks guys.

gojericho0

kriscamaro68 wrote:

Also are the only differences between subnetting and CIDR the fact that one is based on classful ip addressing and CIDR is for the most part not. I know that you use class c ip range to determine how many ip addresses are in what ever / your using (example: /20= 4094 or 256x16-2) but that is the only thing I see it using from the classful method. From what I can tell in subnetting you are stuck with a default subnet mask for a class c where with CIDR you have all the different /x and so on that you can use which lets you use a broad range of subnets depending on your situation. Any help in clearing this up would be appreciated. Thanks guys.

When you use CIDR with an address you are supernetting an IP address. ISP will use this technique to save address space when providing companies with IP address. CIDR occurs when you borrow host bits for networking in a classful address. For example if we had a class C of 195.20.1.0 \24 and turned it to 195.20.0.0 \16 it would be supernetted to provide more networks

It also helps to cut down on the size of a network tables by allowing to advertise one supernet for a series of contiguous networks

Mishra

Don't worry, I wrote all this down just because its a good refresher on my part. Read through it before trying to understand it. The numbers on the charts don't line up to each other...






26 is the number of 1s there are in the binary notation.

/26 = 11111111.11111111.11111111.11000000

Write down your base 2 numbering scheme BEFORE starting the exam. You can do all the math you need before you start the exam to get these numbers written down.

32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1
1 1 0 0 0 0 0 0

You can then add the numbers together corresponding to the 1's in the 4rd octet. If you do enough practice questions then you start to memorize this

128 + 64 = 192

so thats 255.255.192.0

now take 256 minus what you came up with 192. 256 - 192 = 64

The number you got above is the beginning of the 2nd network. So here is the network structure for 192.168.0.0

192.168.0.0 (network)
192.168.0.63 (broadcast)

192.168.0.64 (you see 64 is the start of the 2nd network)
192.168.0.127 (broadcast... Basically 64+63)

192.168.0.128 (network)
192.168.0.191 (broadcast... 128+63)

192.168.0.192 (network)
192.168.0.255 (broadcast)





And to quickly find out how many hosts then you have your 2 ^ X - 2 being that x = how many 0s you have. Use the base 2 numbering chart you wrote down before the exam to figure out what 2 ^ 6 is by finding the 6th number past 1.

128 64 32 16 8 4 2 1
7 6 5 4 3 2 1 0

Networks

2^2 = 4 (you see 2 on the chart is 4)

Hosts

2^6 = 64 - 2 = 62 (you see 6 on the chart is 64)

remyforbes777

Umm you can't have a network address of 192.168.0.256. Not allowed.

Mishra

Thanks. Just a typo.