With the help of this static class, one can authenticate ip by providing, one or more allowed ips, ip pattern or ip CIDR.

ip_auth.php

<?php

/**
Usage:
IpAuth::doAuth(array(
            'xxx.xxx.xxx.* ',
            'yyy.yyy.yyy.* ',
            'aaa.bbb.ccc.0/19',
            'a.b.c.d',
));

*/

class IpAuth {

    public static function doAuth($allowed_ips_pattern=array(),$ip=''){
        if(empty($ip)) $ip = $_SERVER['REMOTE_ADDR'];
        foreach($allowed_ips_pattern as $ip_pattern){
            if(@self::netMatch($ip_pattern, $ip)) return true;
        }
        //log error
        die('IP Not Authorised');

    }

    /**
     * Courtesy:
     * http://stackoverflow.com/questions/10421613/match-ipv4-address-given-ip-range-mask
     */
    public static function netMatch($network, $ip) {
        $network=trim($network);
        $orig_network = $network;
        $ip = trim($ip);
        if ($ip == $network) {
            //echo "used network ($network) for ($ip)\n";
            return TRUE;
        }
        $network = str_replace(' ', '', $network);
        if (strpos($network, '*') !== FALSE) {
            if (strpos($network, '/') !== FALSE) {
                $asParts = explode('/', $network);
                $network = @ $asParts[0];
            }
            $nCount = substr_count($network, '*');
            $network = str_replace('*', '0', $network);
            if ($nCount == 1) {
                $network .= '/24';
            } else if ($nCount == 2) {
                $network .= '/16';
            } else if ($nCount == 3) {
                $network .= '/8';
            } else if ($nCount > 3) {
                return TRUE; // if *.*.*.*, then all, so matched
            }
        }

        //echo "from original network($orig_network), used network ($network) for ($ip)\n";
        $d = strpos($network, '-');
        if ($d === FALSE) {
            $ip_arr = explode('/', $network);
            if (!preg_match("@\d*\.\d*\.\d*\.\d*@", $ip_arr[0], $matches)){
                $ip_arr[0].=".0";    // Alternate form 194.1.4/24
            }
            $network_long = ip2long($ip_arr[0]);
            $x = ip2long($ip_arr[1]);
            $mask = long2ip($x) == $ip_arr[1] ? $x : (0xffffffff << (32 - $ip_arr[1]));
            $ip_long = ip2long($ip);
            return ($ip_long & $mask) == ($network_long & $mask);
        } else {
            $from = trim(ip2long(substr($network, 0, $d)));
            $to = trim(ip2long(substr($network, $d+1)));
            $ip = ip2long($ip);
            return ($ip>=$from and $ip<=$to);
        }
    }
}