How do I compare IP addresses in PHP as binary strings?

Question

I am currently working with IPv4 and IPv6 addresses in a PHP based project, and I need to be able to compare two IPs to determine which one is a higher number. For example, that 192.168.1.9 is greater than 192.168.1.1. In order to do this, I converted the IPs to binary strings using inet_pton and unpack (I am familiar with ip2long, however it is limited to IPv4).

This method seemed to work fine at first, however I soon discovered that I get the incorrect result when I compare any IP ending in .32 to a lower IP address. For example, if I compare 192.168.1.0 to 192.168.1.32, my script tells me that 192.168.1.0 is greater than 192.168.1.32. This only happens when one of the IPs ends in .32. The first three octets of the IP can be changed, and the result is the same.

The following PHP code generates a page that illustrates this issue:

// Loop through every possible last octet, starting with zero
for ($i = 0; $i <= 255; $i++) {

    // Define two IPs, with second IP increasing on each loop by 1
    $IP1 = "192.168.1.0";
    $IP2 = "192.168.1.".$i;

    // Convert each IP to a binary string
    $IP1_bin = current(unpack("A4",inet_pton($IP1)));
    $IP2_bin = current(unpack("A4",inet_pton($IP2)));

    // Convert each IP back to human readable format, just to show they were converted properly
    $IP1_string = inet_ntop(pack("A4",$IP1_bin));
    $IP2_string = inet_ntop(pack("A4",$IP2_bin));

    // Compare each IP and echo the result
    if ($IP1_bin < $IP2_bin) {echo '<p>'.$IP1_string.' is LESS than '.$IP2_string.'</p>';}
    if ($IP1_bin === $IP2_bin) {echo '<p>'.$IP1_string.' is EQUAL to '.$IP2_string.'</p>';}
    if ($IP1_bin > $IP2_bin) {echo '<p>'.$IP1_string.' is GREATER than '.$IP2_string.'</p>';}

    // I have also tried using strcmp for the binary comparison, with the same result
    // if (strcmp($IP1_bin,$IP2_bin) < 0) {echo '<p>'.$IP1_string.' is LESS than '.$IP2_string.'</p>';}
    // if (strcmp($IP1_bin,$IP2_bin) === 0) {echo '<p>'.$IP1_string.' iS EQUAL to '.$IP2_string.'</p>';}
    // if (strcmp($IP1_bin,$IP2_bin) > 0) {echo '<p>'.$IP1_string.' is GREATER than '.$IP2_string.'</p>';}
}

?>

Here is a sample of the result:

192.168.1.0 is EQUAL to 192.168.1.0
192.168.1.0 is LESS than 192.168.1.1
192.168.1.0 is LESS than 192.168.1.2
192.168.1.0 is LESS than 192.168.1.3
192.168.1.0 is LESS than 192.168.1.4
...
192.168.1.0 is LESS than 192.168.1.31
192.168.1.0 is GREATER than 192.168.1.32
192.168.1.0 is LESS than 192.168.1.33
...

Converting the IPs back to human readable format returns the correct IP, so I believe the issue is with the comparison. I have tried switching to strcmp for the binary comparison, however the result was the same.

Any help in determining the cause of this would be greatly appreciated. I am not set on using the IP conversion and comparison methods shown in the example script, however I do need to stick with methods that support both IPv4 and IPv6. Thanks.

I am running PHP verison 5.3.3, with Zend Engine v2.3.0 and ionCube PHP Loader v4.6.1

Edit: I resolved the issue by changing the unpack format from "A4" (space-padded strings) to "a4" (NUL-padded strings). See my answer below for details.

For IPv6, these will be very large binary numbers (too large to convert to integers). I tried casting them as binary with no change to the result. — Seth McCauley, Mar 14 '15 at 22:16
Also, strcmp is intended to compare binary strings. As mentioned above, the result was the same when I tried comparing with this method. — Seth McCauley, Mar 14 '15 at 22:18
Untested - may be useful: [IPvx provides useful functions for IP (v4/v6) address](https://github.com/nov/ipvx/blob/master/IPvx.php). Seems to convert to binary format which may make comparing easier?. — Ryan Vincent, Mar 14 '15 at 22:27
Much appreciated. I would prefer to stick with inet_pton though... this code will be used in production soon, so I'd rather not go with a library I'm not familiar with. — Seth McCauley, Mar 18 '15 at 14:44

score 4 · Accepted Answer · answered Mar 18 '15 at 16:43

After much troubleshooting, I discovered the cause of this issue. When using the unpack function, I had used the format code "A4" which is for space-padded strings. This was causing anything ending with the number 32 to get treated as whitespace, which would then be trimmed. For example, after unpacking the binary value for 192.168.1.32, I converted it to hex. The result was C0A801 but should have been C0A80120. The result was even worse if the IP was 192.32.32.32, as it would trim all the way down to C0 (essentially 192.0.0.0).

Switching to unpack format "a4" (NUL-padded strings) fixed the issue. Now the only IP's that get truncated are the ones ending in .0, which is the expected behavior. I find it odd that this fixed the issue, as nearly every example I've come across for unpacking IPv4 and IPv6 addresses used the format "A4". Perhaps they changed inet_pton to space-padded in a newer version.

score 1 · Answer 2 · answered Mar 15 '15 at 00:15

1

If you would like to convert to a number instead of binary below a couple of useful functions:

function ipv6ToNum($ip)
{
    $binaryNum = '';
    foreach (unpack('C*', inet_pton($ip)) as $byte) {
        $binaryNum .= str_pad(decbin($byte), 8, "0", STR_PAD_LEFT);
    }
    $binToInt = base_convert(ltrim($binaryNum, '0'), 2, 10);

    return $binToInt;
}

function ipv4ToNum($ip)
{
    $result = 0;
    $ipNumbers = explode('.', $ip);
    for ($i=0; $i < count($ipNumbers); $i++) {
        $power = count($ipNumbers) - $i;
        $result += pow(256, $i) * $ipNumbers[$i];
    }

    return $result;
}

(I repeat the ip2long logic in the ipv4ToNum)

answered Mar 15 '15 at 00:15

nik.longstone

244
2
8

Actually the function doesn't check if the ipv6 format is correct, but the result of the function can be considered as correct, 0:0:0:0:0:0:0:1 = 1. In PHP also that evaluation will compare a string like ipv6ToNum(':::2') === '2' that is also valid. – nik.longstone Mar 15 '15 at 00:32
Thank you for the response, but I would prefer to debug the results of the inet_pton function as I use it in many places and I need the data to be binary. inet_pton is commonly used for working with IPv6 in PHP, so I was hoping someone had come across this issue before. – Seth McCauley Mar 18 '15 at 14:41

How do I compare IP addresses in PHP as binary strings?

2 Answers2