IPv4 CIDR Calculator

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CIDR stands for Classless Inter-Domain Routing.

IPv4 CIDR notation: [IPv4 network address]/[host identifier].

For example, 192.168.0.1/24.

The Calculation

Let's calculate this 192.168.0.1/24

Break it down, we have two parts:

  • 192.168.0.1 and
  • /24.

The 192.168.0.1 is an IPv4 dot-decimal notation. The network address.

The /24 is the subnet mask as a host identifier (to denote its own network).

This is a good explanation.

This is the main line of how the tool above does the calculation.

Steps:

  1. Convert the IPv4 dot-decimal notation into a dot-binary notation.

    192.168.0.1 ► 11000000.10101000.00000000.00000001

  2. Translate the subnet mask suffix — in above example: /24 — into a dot-binary notation.

    The total length of the notation is 32 bits (32 of 0s and / or 1s).

    Look at the number after the slash, it is 24. Meaning, we create 24 of 1s from the left (24 leading 1s).

    The last 8 bits (32 - 24 = 8) are filled with 0s.

    Now we have the binary version of the suffix.

    Put dot (.) to separate each segment with 8-bit interval (dot-binary notation).

    /24 becomes 11111111111111111111111100000000

    Then we put dots:

    11111111.11111111.11111111.00000000

  3. Then we do the AND operation for the binary version of the IPv4 and the binary version of the subnet mask.

    AND OPERATIONS:

    • 1 AND 1 = 1
    • 1 AND 0 = 0
    • 0 AND 1 = 0
    • 0 AND 0 = 0

    IP: 11000000.10101000.00000000.00000001    

    Mask: 11111111.11111111.11111111.00000000    

    -------------------------------------------------------------- AND

    RESULT: 11000000.10101000.00000000.00000000    

  4. Afterwards, convert the AND result back to dot-decimal notation. This is the first IP within the block. The HOST address.

    11000000.10101000.00000000.00000000 ► 192.168.0.0

  5. We now need to get the last IP within the block. To do that, first convert the subnet mask dot-binary notation into a dot-decimal notation.

    11111111.11111111.11111111.00000000 ► 255.255.255.0

  6. Then, do the subtraction as follows:

    FULL MASK: 255.255.255.255  

    SUBNET MASK: 255.255.255.  0  

    --------------------------- -

    SUBTRACTION RESULT:   0.  0.  0.255  

    Or, invert the subnet mask binary version, then convert it back to decimal.

    11111111.11111111.11111111.00000000

    Invert it (1 to 0 and 0 to 1):

    00000000.00000000.00000000.11111111

    00000000.00000000.00000000.11111111 to decimal is 0.0.0.255.

    Same result. ✅

  7. The last IP then is calculated by adding the subtraction result from the sixth step with the first IP we have from the fourth step.

    STARTING IP: 192.168.  0.  0  

    SUBTRACTION RESULT:   0.  0.  0.255  

    --------------------------- +

    LAST IP: 192.168.  0.255  

  8. Complete. ✅

    So then 192.168.0.1/24 is to tell the computer that its network HOST is at 192.168.0.0 and the network block is within 192.168.0.0 to 192.168.0.255.

That's the idea for translating the /number suffix (network block) from an IPv4 CIDR notation. It's not limited to this explanation, but the principle is usually rather similar to that.