Today is Tuesday, January 21, 2025

CIDR

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:

b₃₁b₃₀b₂₉b₂₈b₂₇b₂₆b₂₅b₂₄.b₂₃b₂₂b₂₁b₂₀b₁₉b₁₈b₁₇b₁₆.b₁₅b₁₄b₁₃b₁₂b₁₁b₁₀b₉b₈.b₇b₆b₅b₄b₃b₂b₁b₀

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 127.0.0.0 or 10.0.0.1.
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 32.0.0.0 to 32.255.255.255 then:
- start IP = 00100000.00000000.00000000.00000000 = 32.0.0.0
- end IP =  00100000.11111111.11111111.11111111 = 32.255.255.255