How to Verify IP Address Inclusion in a CIDR Subnet?

Verifying IP Address Inclusion in a CIDR Subnet

To determine whether an IPv4 address falls within a specified CIDR subnet, a straightforward method involves the following steps:

Conversion to Long Integers:

• Utilize the ip2long() function to convert both the IP address and the subnet range into long integers.

Subnet Mask Derivation:

• If the subnet mask is not explicitly provided (/xx), assume a default mask of /32.
• Compute the subnet mask by applying a left shift of 1s to 32 minus the mask length.

Bitwise Comparison:

• Perform a bitwise AND operation between the IP long integer and the subnet mask.
• Check if the result is equal to the subnet long integer.

Implementation:

The following PHP function encapsulates this logic:

<code class="php">function cidr_match($ip, $range)
{
    list ($subnet, $bits) = explode('/', $range);
    if ($bits === null) {
        $bits = 32;
    }
    $ip = ip2long($ip);
    $subnet = ip2long($subnet);
    $mask = -1 << (32 - $bits);
    $subnet &= $mask;
    return ($ip & $mask) == $subnet;
}</code>

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