diff --git a/assets/App.js b/assets/App.js
index 4ffa2b1..8cb2948 100644
--- a/assets/App.js
+++ b/assets/App.js
@@ -35,6 +35,10 @@ App.auth = async function () {
     this.fillNetworks();
 
     $("#panel").show();
+
+    let invalidNetworks = await JSONRPC.__invoke("getInvalidNetworks");
+
+    $("body").append(`<span>There are invalid networks ` +invalidNetworks.join("<BR>")+` </span>`);
 }
 
 App.fillNetworks = function () {
diff --git a/classes/IPv4Subnet.php b/classes/IPv4Subnet.php
new file mode 100644
index 0000000..467907b
--- /dev/null
+++ b/classes/IPv4Subnet.php
@@ -0,0 +1,46 @@
+<?php
+
+class IPv4Subnet
+{
+
+    private string $address;
+    private int $prefix;
+
+    /**
+     * @param string $subnetAddress
+     * @param int $prefix
+     */
+    public function __construct(string $subnetAddress, int $prefix)
+    {
+        $this->address = $subnetAddress;
+        if (ip2long($subnetAddress) === false) {
+            throw new RuntimeException("Invalid subnet address: " . $subnetAddress);
+        }
+        $this->prefix = $prefix;
+        if ($prefix < 0 or $prefix > 32) {
+            throw new RuntimeException("Invalid subnet prefix: " . $prefix);
+        }
+    }
+
+    public function getFirstAddress()
+    {
+        $a = ip2long($this->address);
+        $mask = ip2long($this->getNetMask());
+        return long2ip($a & $mask);
+    }
+
+    public function getLastAddress()
+    {
+        return long2ip(ip2long($this->getFirstAddress()) + $this->getAddressCount() - 1);
+    }
+
+    public function getAddressCount()
+    {
+        return pow(2, 32 - $this->prefix);
+    }
+
+    public function getNetMask()
+    {
+        return long2ip(-1 << (32 - $this->prefix));
+    }
+}
\ No newline at end of file
diff --git a/classes/RPC.php b/classes/RPC.php
index 113d150..7acc246 100644
--- a/classes/RPC.php
+++ b/classes/RPC.php
@@ -88,6 +88,20 @@ class RPC
         return (new NetworkConfigReader())->getConfigs();
     }
 
+    public function getInvalidNetworks(): array
+    {
+        $this->checkAuth();
+        $invalid = [];
+        foreach ((new NetworkConfigReader())->getConfigs() as $config) {
+            foreach ($config["networks"] as $network) {
+                if (!RouteUtil::validateSubnet($network)) {
+                    $invalid[] = $network;
+                }
+            }
+        }
+        return $invalid;
+    }
+
 
     public function logout(): void
     {
diff --git a/classes/RouteUtil.php b/classes/RouteUtil.php
new file mode 100644
index 0000000..db9a83a
--- /dev/null
+++ b/classes/RouteUtil.php
@@ -0,0 +1,18 @@
+<?php
+
+class RouteUtil
+{
+    public static function validateSubnet($subnet)
+    {
+        $parts = explode("/", $subnet);
+        if (count($parts) != 2) {
+            return false;
+        }
+        try {
+            $ipv4subnet = new IPv4Subnet($parts[0], $parts[1]);
+            return $parts[0] == $ipv4subnet->getFirstAddress();
+        } catch (Exception $e) {
+            return false;
+        }
+    }
+}
\ No newline at end of file