Script to get an example network for use in test setups.

example-network-v4.php

<?php

/**
 * @param integer $prefix CIDR prefix (24..32).
 * @return array
 */
function example(int $prefix): array
{
    // Networks for documentation (RFC 5737)
    $source = ['192.0.2.0', '198.51.100.0', '203.0.113.0'];
    $index = array_rand($source, 1);

    return network($source[$index].'/'.$prefix);
}

/**
 * @param string $network Network CIDR.
 * @return array
 */
function network(string $network): array
{
    list($ip, $prefix) = explode('/', $network, 2);

    $min = ip2long($ip);
    $size = 1 << (32 - $prefix);
    $range = range($min, $min + 255, $size);
    $index = array_rand($range, 1); 

    return result($range[$index], $size);
}

/**
 * @param integer $start First IP of the network (numeric).
 * @param integer $size Number of IPs in the network.
 * @return array
 */
function result(int $start, int $size): array
{
    $data['ip']['first']  = $start;
    $data['ip']['last']   = $start + $size;
    $data['num']['first'] = long2ip($start);
    $data['num']['last']  = long2ip($start + $size);
    $data['cidr'] = long2ip($start).'/'.(32 - log($size, 2));

    return $data;
}

// example usage
echo json_encode(example(27), JSON_PRETTY_PRINT|JSON_UNESCAPED_SLASHES);