diff --git a/includes/functions.php b/includes/functions.php
index 4d299314..c58b2b13 100755
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -452,3 +452,30 @@ function getBridgedState()
     return  $arrHostapdConf['BridgedEnable'];
 }
 
+/**
+ * Validates the format of a CIDR notation string
+ *
+ * @param string $cidr
+ * @return bool
+ */
+function validateCidr($cidr)
+{
+    $parts = explode('/', $cidr);
+    if(count($parts) != 2) {
+        return false;
+    }
+    $ip = $parts[0];
+    $netmask = intval($parts[1]);
+
+    if($netmask < 0) {
+        return false;
+    }
+    if(filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)) {
+        return $netmask <= 32;
+    }
+    if(filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6)) {
+        return $netmask <= 128;
+    }
+    return false;
+}
+