PHP class for working IP addresses.

<?php

class cidr
{
	// convert cidr to netmask
	// e.g. 21 = 255.255.248.0
	public function cidr2netmask($cidr)
	{
		for ($i = 1; $i <= 32; $i ++)
			$bin .= $cidr >= $i ? '1' : '0';
		
		$netmask = long2ip(bindec($bin));
		
		if ($netmask == "0.0.0.0")
			return false;
		
		return $netmask;
	}
	
	// get network address from cidr subnet
	// e.g. 10.0.2.56/21 = 10.0.0.0
	public function cidr2network($ip, $cidr)
	{
		$network = long2ip((ip2long($ip)) & ((- 1 << (32 - (int) $cidr))));
		
		return $network;
	}
	
	// convert netmask to cidr
	// e.g. 255.255.255.128 = 25
	public function netmask2cidr($netmask)
	{
		$bits = 0;
		$netmask = explode(".", $netmask);
		
		foreach ($netmask as $octect)
			$bits += strlen(str_replace("0", "", decbin($octect)));
		
		return $bits;
	}
	
	// is ip in subnet
	// e.g. is 10.5.21.30 in 10.5.16.0/20 == true
	//      is 192.168.50.2 in 192.168.30.0/23 == false
	public function cidr_match($ip, $network, $cidr)
	{
		if ((ip2long($ip) & ~ ((1 << (32 - $cidr)) - 1)) == ip2long($network))
		{
			return true;
		}
		
		return false;
	}
}

?>