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CIDR Print E-mail
Written by Administrator   
Thursday, 14 June 2012 21:28

Today is Saturday, June 22, 2024


CIDR stands for Classless Inter-Domain Routing.
CIDR is a new addressing scheme for the Internet which allows for more efficient allocation of IP addresses than the old Class A, B, and C address scheme.

With a new network being connected to the Internet every 30 minutes, the Internet was faced with two critical problems:
- Running out of IP addresses
- Running out of capacity in the global routing tables

An IP address is actually a binary number 32 bits long. To make it "more readable" we split the 32 bits into 4 groups of 8 bits as follows:


The first 8 bits are used to indicate the starting IP number.
The remaining 24 bits (32 -8) are used to determine ending IP number. All of the 24 remaining bits can be either 1's or 0's which is the same as 2 to the power of 24 numbers in range.

An IPV4 address we are all familiar with looks like so XXX.XXX.XXX.XXX where XXX is a number between 0 and 255. For example or
This address is the decimal equivalent of the 4 groups of 8 bits.

As stated previously, each bit in the group can have the value of 1 (if it exists) or 0 (if it does not exist) as shown below:

If b7b6b5b4b3b2b1b0 = 11111111 then this equals 27+26+25+ 24+23+22+ 21+20 = 128+64+32+16+8+4+2+1 = XXX = 255

If b7b6b5b4b3b2b1b0 = 00100000 then this equals 0+0+25+0+0+0+0+0 = 0+0+32+0+0+0+0+0 = XX = 32

If we had the IP range of to then:
- start IP = 00100000.00000000.00000000.00000000 =
- end IP = 00100000.11111111.11111111.11111111 =

Last Updated on Saturday, 11 August 2012 18:28

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