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<?php // simple example $bcast = ip2long("192.168.178.255"); $smask = ip2long("255.255.255.0"); $nmask = $bcast & $smask; echo long2ip($nmask); // Will give 192.168.178.0 ?> With this example you are able to check if a given host is in your own local net or not (on linux): <?php /** * Check if a client IP is in our Server subnet * * @param string $client_ip * @param string $server_ip * @return boolean */ function clientInSameSubnet($client_ip=false,$server_ip=false) { if (!$client_ip) $client_ip = $_SERVER['REMOTE_ADDR']; if (!$server_ip) $server_ip = $_SERVER['SERVER_ADDR']; // Extract broadcast and netmask from ifconfig if (!($p = popen("ifconfig","r"))) return false; $out = ""; while(!feof($p)) $out .= fread($p,1024); fclose($p); // This is because the php.net comment function does not // allow long lines. $match = "/^.*".$server_ip; $match .= ".*Bcast:(\d{1,3}\.\d{1,3}i\.\d{1,3}\.\d{1,3}).*"; $match .= "Mask:(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})$/im"; if (!preg_match($match,$out,$regs)) return false; $bcast = ip2long($regs[1]); $smask = ip2long($regs[2]); $ipadr = ip2long($client_ip); $nmask = $bcast & $smask; return (($ipadr & $smask) == ($nmask & $smask)); } ?>