Is public IP address?

Question

I would like to know whether there is any easy method to check whether an IP address is publicly accessible or is private.

More specifically. I know for example, that 127.0.0.1 is a private IP address that point into the same machine, 255.255.255.255 if for broadcasting into the same network, 192.168.1.0 is for local network addresses and so on. But how can I distinguish whether a given IP address is not one of the private IP and is publicly accessible?

Answer 1

http://en.wikipedia.org/wiki/Private_network lists the various ranges. Just construct an if statement.

Answer 2

Pulic/private IPv4 addresses are defined in RFC 5735 . In short:

• 0.0.0.0/8 is invalid
• 127.0.0.0/8 is localhost
• 169.254.0.0/16 is an unconfigured IP. You should treat it like a local address
• 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 are private networks
• 224.0.0.0/4 is multicast
• Everything else is publicly reachable, or reserved

For IPv6, refer to RFC 5165 . In short:

• ::/128 is the unspecified address, ::1/128 is localhost
• ::ffff:0:0/96 are IPv4-mapped addresses
• fe80::/10 and fc00::/7 are private networks
• ff00::/8 is multicast
• Everything else is publicly reachable, or reserved

Note that services on machines without a public IP may still be reachable from the internet with the help of port forwarding or other firewall rules.

Answer 3

function validateIpAddress($ip_addr)
{
    $result = true;
    if(preg_match("/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/",$ip_addr))
    {
        $parts=explode(".",$ip_addr);
        foreach($parts as $ip_parts)
        {
            if(intval($ip_parts)>255 || intval($ip_parts)<0)
                $result=false;
        }
        if (intval($parts[0])==0 || intval($parts[0])==10 || intval($parts[0])==127 || (intval($parts[0])>223 && intval($parts[0])<240))
        {
            $result=false;
        }
        if ((intval($parts[0])==192 && intval($parts[1])==168) || (intval($parts[0])==169 && intval($parts[1])==254))
        {
            $result=false;
        }

        if (intval($parts[0])==172 && intval($parts[1])>15 && intval($parts[1])<32 )
        {
            $result=false;
        }
    }
    else
    {
        $result = false; //if format of ip address doesn't matches 
    }
    return $result;
}

Answer 4

One solution is mentioned by Ed Heal, but there is another one:

Just connect to some external host and ask it for the IP it sees, like that (example for PHP):

$my_public_ip = file_get_contents('http://ip.42.pl/raw');

This specific example I know will return single string containing only an IP address. I do not know other services offering this, although there are probably plenty of them. The main page of the above script / service is: http://ip.42.pl/ .

If you know similar services, please post their URLs in the comments, so future readers have other options.