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User Contributed Notes

David GASTALDIN
Here a IP-Range to CIDRs function that I wrote for the purpose of filling my Postfix client.cidr with ripe-ncc data to block spamming from useless countries. Strcmp functions are meant to work around the silly PHP string comparison which inevitably tries compare strings as numbers when possible. I'll make no comment about that fact ... bit I have to bite my tong hard :

function PlageVersCIDRs($ip_min, $ip_max) {
    $cidrs = array();
    $ip_min_bin = sprintf('%032b', $ip_min);
    $ip_max_bin = sprintf('%032b', $ip_max);
    $ip_cour_bin = $ip_min_bin;
    while (strcmp($ip_cour_bin, $ip_max_bin) <= 0) {
        $lng_reseau = 32;
        $ip_reseau_bin = $ip_cour_bin;
        while (($ip_cour_bin[$lng_reseau - 1] == '0') && (strcmp(substr_replace($ip_reseau_bin, '1', $lng_reseau - 1, 1), $ip_max_bin) <= 0)) {
            $ip_reseau_bin[$lng_reseau - 1] = '1';
            $lng_reseau--;
        }
        $cidrs[] = long2ip(bindec($ip_cour_bin)).'/'.$lng_reseau;
        $ip_cour_bin = sprintf('%032b', bindec($ip_reseau_bin) + 1);
    }
    return $cidrs;
}
claudiu at cnixs dot com
A simple and very fast function to check against CIDR.

Your previous examples are too complicated and involves a lot of functions call.

Here it is (only with arithmetic operators and call only to ip2long () and split() ):
<?php
  function ipCIDRCheck ($IP, $CIDR) {
    list ($net, $mask) = split ("/", $CIDR);
   
    $ip_net = ip2long ($net);
    $ip_mask = ~((1 << (32 - $mask)) - 1);

    $ip_ip = ip2long ($IP);

    $ip_ip_net = $ip_ip & $ip_mask;

    return ($ip_ip_net == $ip_net);
  }
?>
call example: <?php echo ipCheck ("192.168.1.23", "192.168.1.0/24"); ?>
dand at ddchosting.com
When I was working on a project I ran into this problem with redirecting. My solution is as follows:
header("Refresh: 5; url=../main/main.php?".session_id()."");

This allowed me to pass the session_id() which is used throughout site to make sure user has loged in.
I hope this helps!
metator at netcabo dot pt
Ups. The function has a bug, though the example still works with it. Just replace the $_POST variables by the function parameters.

<?
function isIPIn($ip, $net, $mask) {
   //doesn't check for the return value of ip2long
   $ip = ip2long($ip);
   $rede = ip2long($net);
   $mask = ip2long($mask);
 
   //AND
   $res = $ip & $mask;
 
   return ($res == $rede);
}
?>
metator at netcabo dot pt
Regarding samuele's note:

You can get faster code if you apply directly what happens in  network devices, such as routers. If you AND (logic operation) the remote ip against the local netmask the result will be the network ip if the remote ip is from the local network. Example:

    192.168.0.16 = 11000000.10101000.00000000.00010000
& 255.255.255.0 = 11111111.11111111.11111111.00000000
--------------------------------------------------------------
   192.168.0.0 = 11000000.10101000.00000000.00000000

And now the code. My example uses a html form where you place the values you want to test:

<HTML><HEAD><TITLE>Check IP</TITLE>
</HEAD><BODY>
    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">

Hope you find this useful.
        IP to check: <input type="text" name="ip"> <br>
        Local network ip: <input type="text" name="net"> <br>
        Local netmask: <input type="text" name="mask"> <br>
        <input type="submit" name="check" value="Check it!">
    </form>
    <?php

/**
 * @param string $ip IP to check in dotted decimal format
 * @param string $net Network IP in dotted decimal format
 * @param string $mask Netmask in dotted decimal format
 * @returns true if the ip belongs to the network, false otherwise
 **/
function isIPIn($ip, $net, $mask) {
    //doesn't check for the return value of ip2long
    $ip = ip2long($_POST['ip']);
    $rede = ip2long($_POST['net']);
    $mask = ip2long($_POST['mask']);
   
    //AND
    $res = $ip & $mask;
   
    return ($res == $rede);
}

if (isset($_POST['check'])) {
     echo isIPIn($_POST['ip'], $_POST['net'], $_POST['mask']) ? "IP IN.": "IP OUT.";
}
 ?>
</BODY><HTML>
nexxer at rogers dot com
In Trevor Hemsley's translation of the perl range2cidr function, the

while ($end > $start)

condition should be

while ($end >= $start)

otherwise it won't work for /32s, ie if you feed range2cidr("1.2.3.4", "1.2.3.4").

-- nex
samuele at norsam dot org
To find if an IP is in a net/mask (very fast):
<?php
function isIPIn($ip,$net,$mask) {
   $lnet=ip2long($net);
   $lip=ip2long($ip);
   $binnet=str_pad( decbin($lnet),32,"0","STR_PAD_LEFT" );
   $firstpart=substr($binnet,0,$mask);
   $binip=str_pad( decbin($lip),32,"0","STR_PAD_LEFT" );
   $firstip=substr($binip,0,$mask);
   return(strcmp($firstpart,$firstip)==0);
   }
?>

This function can be compacted, avoiding some variable settings, but the function will not be too clear to read...
Example code, used to made a kind of location service network-based:

<?php
$n = array ( "192.168.0.0/16"   => "TUSCANY",
             "192.168.1.0/24"   => "- Florence",
             "192.168.2.0/24"   => "- Pisa",
             "192.168.3.0/24"   => "- Siena",
             "192.168.64.0/21" => "- Tuscan Archipelago",
             "192.168.64.0/23" => "--- Elba Island",
             "192.168.66.0/24" => "--- Capraia Island",
             "192.168.67.0/24" => "--- Giannutri Island");

// Normally you should use the following line
$myip = $HTTP_SERVER_VARS['REMOTE_ADDR'];
// This is first example: returns Tuscany/Pisa
$myip = "192.168.2.33";
// This is second example: returns Tuscany/T.Arch./Elba
$myip = "192.168.65.34";

echo "Your position:<br />\n";
foreach ( $n as $k=>$v ) {
   list($net,$mask)=split("/",$k);
   if (isIPIn($myip,$net,$mask)) {
       echo $n[$k]."<br />\n"; }
   }
?>

and so on...
anderson at piq dot com dot br
If you want to get the interface of an IP, based on the local route table, use this.

function GetIfaceforIP($user_ip)
{
    $route = "/bin/netstat -rn";

    exec($route, $aoutput);
    foreach($aoutput as $key => $line)
    {
        if($key > 1)
        {
            $line = ereg_replace("[[:space:]]+",",",$line);
            list($network, $gateway, $mask, $flags, $mss, $window, $irtt, $iface) = explode(",", $line)
            if((ip2long($user_ip) & ip2long($mask)) == ip2long($network))
            {
                return $iface;
            }
        }
    }
}
Alternative cidr_conv function - a little easier to follow

function cidr_conv($cidr_address) {
  $first = substr($cidr_address, 0, strpos($cidr_address, "/"));
  $netmask = substr(strstr($cidr_address, "/"), 1);

  $first_bin = str_pad(decbin(ip2long($first)), 32, "0", STR_PAD_LEFT);
  $netmask_bin = str_pad(str_repeat("1", (integer)$netmask), 32, "0", STR_PAD_RIGHT);
 
  for ($i = 0; $i < 32; $i++) {
    if ($netmask_bin[$i] == "1")
      $last_bin .= $first_bin[$i];
    else
      $last_bin .= "1";
  }

  $last = long2ip(bindec($last_bin));

  return "$first - $last";
}
trevor-hemsley at nospam dot dial dot pipex dot com
Previous example of IP range to CIDR list does not cope with ranges as well as the perl Net::Netmask range2cidrlist() function. In PHP this looks like

<?
function imask($this)
{
// use base_convert not dechex because dechex is broken and returns 0x80000000 instead of 0xffffffff
return base_convert((pow(2,32) - pow(2, (32-$this)))), 10, 16);
}

function imaxblock($ibase, $tbit)
{
while ($tbit > 0)
{
$im = hexdec(imask($tbit-1));
$imand = $ibase & $im;
if ($imand != $ibase)
{
break;
}
$tbit--;
}
return $tbit;
}

function range2cidrlist($istart, $iend)
{
// this function returns an array of cidr lists that map the range given
$s = explode(".", $istart);
// PHP ip2long does not handle leading zeros on IP addresses! 172.016 comes back as 172.14, seems to be treated as octal!
$start = "";
$dot = "";
while (list($key,$val) = each($s))
{
$start = sprintf("%s%s%d",$start,$dot,$val);
$dot = ".";
}
$end = "";
$dot = "";
$e = explode(".",$iend);
while (list($key,$val) = each($e))
{
$end = sprintf("%s%s%d",$end,$dot,$val);
$dot = ".";
}
$start = ip2long($start);
$end = ip2long($end);
$result = array();
while ($end > $start)
{
$maxsize = imaxblock($start,32);
$x = log($end - $start + 1)/log(2);
$maxdiff = floor(32 - floor($x));
$ip = long2ip($start);
if ($maxsize < $maxdiff)
{
$maxsize = $maxdiff;
}
array_push($result,"$ip/$maxsize");
$start += pow(2, (32-$maxsize));
}
return $result;
}
?>
philippe-at-cyberabuse.org
... and this one will do the opposite (o return NULL for invalid netblocks) :

1.0.0.0 1.0.255.255 -> 1.0.0.0/16
1.0.0.0 1.3.255.255 -> 1.0.0.0/14
192.168.0.0 192.168.0.255 -> 192.168.0.0/24

function ip2cidr($ip_start,$ip_end) {
if(long2ip(ip2long($ip_start))!=$ip_start or long2ip(ip2long($ip_end))!=$ip_end) return NULL;
$ipl_start=(int)ip2long($ip_start);
$ipl_end=(int)ip2long($ip_end);
if($ipl_start>0 && $ipl_end<0) $delta=($ipl_end+4294967296)-$ipl_start;
else $delta=$ipl_end-$ipl_start;
$netmask=str_pad(decbin($delta),32,"0","STR_PAD_LEFT");
if(ip2long($ip_start)==0 && substr_count($netmask,"1")==32) return "0.0.0.0/0";
if($delta<0 or ($delta>0 && $delta%2==0)) return NULL;
for($mask=0;$mask<32;$mask++) if($netmask[$mask]==1) break;
if(substr_count($netmask,"0")!=$mask) return NULL;
return "$ip_start/$mask";
}
philippe-at-cyberabuse.org
PHP miss CIDR functions.

This one will convert a CIDR like this:
0.0.0.0/16 -> 0.0.0.0 - 0.0.255.255
127.0/16 -> 127.0.0.0 - 127.0.255.255
etc...

function cidrconv($net) {
$start=strtok($net,"/");
$n=3-substr_count($net, ".");
if ($n>0) { for ($i=$n;$i>0;$i--) $start.=".0"; }
$bits1=str_pad(decbin(ip2long($start)),32,"0","STR_PAD_LEFT");
$net=pow(2,(32-substr(strstr($net,"/"),1)))-1;
$bits2=str_pad(decbin($net),32,"0","STR_PAD_LEFT");
for ($i=0;$i<32;$i++) {
if ($bits1[$i]==$bits2[$i]) $final.=$bits1[$i];
if ($bits1[$i]==1 and $bits2[$i]==0) $final.=$bits1[$i];
if ($bits1[$i]==0 and $bits2[$i]==1) $final.=$bits2[$i];
}
return $start." - ".long2ip(bindec($final));
}