What is the calculation for converting an IP address to CIDR?

# What is the calculation for converting an IP address to CIDR?

• sabato 17 marzo 2012 21:01

Hello,

My MCTS book explains about "CIDR" ("Classless Inter-Domain Routing"), thus:

For example "/24".

The "CIDR" form shows the number of "bits" turned on in the "Subnet Mask":

255.255.224.0 is:

11111111.11111111.11100000.00000000, equalling 19 bits turned on, or "/19".

Unfortunately the book does not explain how you do the calculation.

I understand that 255 adds up to 8 turned-on bits.

However, what is the calculation that I need to know in order to convert that 224 figure into the 3 turned-on bits?

Robert339

Ps. I have tried inserting carrier returns to make spaces between the paragraphs in this message, but they are not saved.

• Modificato sabato 17 marzo 2012 21:01
• Modificato sabato 17 marzo 2012 21:03
• Modificato sabato 17 marzo 2012 21:04
• Modificato sabato 17 marzo 2012 21:08
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### Tutte le risposte

• domenica 18 marzo 2012 19:27

224 is equivalent to 128+64+32= 2^7+2^6+2^5 and 11100000 in binary.

19 is number of ones from left to right (8+8+3) that mask first 19 digits of IP address and the remnant digits are not taken into count because of multiplication by zeros.

More on this subject see http://tools.ietf.org/html/rfc4632

Regards

Milos

• domenica 18 marzo 2012 23:13

Hello Milos,

Thank you for the information. You say the "224" is equivalent to 128+64+32, but I am afraid I am missing something here. How did you calculate those figures?

What I am looking for is the method for answering the following example question in an examination:

What is the CIDR equivalent for 255.255.255.224?

(MCTS 70:680 Assessment Test - Windows Configuration Study Guide by William Panek.)

Regards,

Robert339

• Modificato domenica 18 marzo 2012 23:15
• Modificato domenica 18 marzo 2012 23:16
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• lunedì 19 marzo 2012 04:29

Everyone has different way of thinking but for exam purposes a table is a good way to go.

Just keep adding from left to right and ask a question if the current sum is bigger then 224 if it is then write down 0 and if it is not write down 1.

 bits 8 7 6 5 4 3 2 1 value 128 64 32 16 8 4 2 1 255 binary 1 1 1 0 0 0 0 0 128 64 32 0 0 0 0 0 224

• Modificato lunedì 19 marzo 2012 04:40
• Contrassegnato come risposta lunedì 19 marzo 2012 18:49
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• lunedì 19 marzo 2012 18:46

Hello Brano,

Thank you. I hope I have understood how this must be done now. In case this may assist anyone else , this is how I assume we have to proceed:

What is the CIDR equivalent for 255.255.255.224?

 bits 8 7 6 5 4 3 2 1 binary 1 1 1 1 1 1 1 1 value 128 64 32 16 8 4 2 1 255 binary 1 1 1 0 0 0 0 0 value 128 64 32 0 0 0 0 0 224

To answer the above question in an examination it is necessary to learn the “Value” figures shown above, and then use them as follows:

An Octet of 255 value, always amounts to 8 bits.

So of our example subnet mask 255.255.255.224

We know that

255.255.255

converts to:

8 + 8 + 8 = 24 (24 bits).

This leaves (in this example) an Octet containing “224” value, requiring conversion as follows:

From your Table, keep adding the highest Values to each other (left to right).

128 + 64 + 32 = 224.

The number of “Value” figures you needed to add together to reach 224, was 3 (bits).

Now add these 3 bits to those of the rest of the subnet mask address (which was 24), to reach your CIDR total, thus:

24 + 3 = 27

255.255.255.224

= 8 + 8 + 8 + 3 = 27

CIDR = /27

• Modificato lunedì 19 marzo 2012 18:47
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