diff --git a/echoip.php b/echoip.php
index 4a724ff..1e5218e 100644
--- a/echoip.php
+++ b/echoip.php
@@ -1,20 +1,34 @@
 <?php
+	function unmask_ipv4($addr) {
+		$prefix = '00000000000000000000ffff';
+		$prefix_bin = hex2bin($prefix);
+		if (($addr_bin = inet_pton($addr)) === FALSE)
+			return false;
+		if (substr($addr_bin, 0, strlen($prefix_bin)) == $prefix_bin)
+			$addr_bin = substr($addr_bin, strlen($prefix_bin));
+		return inet_ntop($addr_bin);
+	}
+
 	ini_set('default_charset', 'iso-8859-1');
 	header('Content-Type: text/plain');
-	$ip = $_SERVER['REMOTE_ADDR'];
-	if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)) {
+	if ($ip = unmask_ipv4($_SERVER['REMOTE_ADDR'])) {
 		$cmd = isset($_GET['cmd']) ? $_GET['cmd'] : '';
-		if ($cmd === 'long') {
-			printf('%u', ip2long($ip));
-		}
-		elseif ($cmd === 'hostname') {
-			echo gethostbyaddr($ip);
-		}
-		else {
-			echo $ip;
+		switch ($cmd) {
+			case 'long':
+				if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4))
+					printf('%u', ip2long($ip));
+				else
+					echo $ip;
+				break;
+			case 'host':
+			case 'hostname':
+				echo gethostbyaddr($ip);
+				break;
+			default:
+				echo $ip;
 		}
 	}
 	else {
-		echo $ip;
+		echo 'Invalid IP address';
 	}
 ?>