diff --git a/app/Rules/ValidIpOrCidr.php b/app/Rules/ValidIpOrCidr.php
new file mode 100644
index 0000000..d72114c
--- /dev/null
+++ b/app/Rules/ValidIpOrCidr.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace App\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+class ValidIpOrCidr implements ValidationRule
+{
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        $v = is_string($value) ? trim($value) : '';
+
+        if (filter_var($v, FILTER_VALIDATE_IP) !== false) {
+            return;
+        }
+
+        if (! str_contains($v, '/')) {
+            $fail(__('settings.lp_whitelist_invalid', ['value' => $v]));
+
+            return;
+        }
+
+        [$ip, $mask] = explode('/', $v, 2);
+        $max = str_contains($ip, ':') ? 128 : 32;
+        if (filter_var($ip, FILTER_VALIDATE_IP) === false || ! ctype_digit($mask) || (int) $mask < 0 || (int) $mask > $max) {
+            $fail(__('settings.lp_whitelist_invalid', ['value' => $v]));
+        }
+    }
+}
diff --git a/lang/de/settings.php b/lang/de/settings.php
index a54c7fa..0336428 100644
--- a/lang/de/settings.php
+++ b/lang/de/settings.php
@@ -63,4 +63,7 @@ return [
 
     // Page title
     'title' => 'Einstellungen — Clusev',
+
+    // IP/CIDR whitelist validation
+    'lp_whitelist_invalid' => ':value ist keine gültige IP/CIDR.',
 ];
diff --git a/lang/en/settings.php b/lang/en/settings.php
index 871eb4d..d4fee99 100644
--- a/lang/en/settings.php
+++ b/lang/en/settings.php
@@ -63,4 +63,7 @@ return [
 
     // Page title
     'title' => 'Settings — Clusev',
+
+    // IP/CIDR whitelist validation
+    'lp_whitelist_invalid' => ':value is not a valid IP/CIDR.',
 ];
diff --git a/tests/Feature/ValidIpOrCidrTest.php b/tests/Feature/ValidIpOrCidrTest.php
new file mode 100644
index 0000000..2fec5bf
--- /dev/null
+++ b/tests/Feature/ValidIpOrCidrTest.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace Tests\Feature;
+
+use App\Rules\ValidIpOrCidr;
+use Illuminate\Support\Facades\Validator;
+use Tests\TestCase;
+
+class ValidIpOrCidrTest extends TestCase
+{
+    private function passes(string $value): bool
+    {
+        return Validator::make(['v' => $value], ['v' => [new ValidIpOrCidr]])->passes();
+    }
+
+    public function test_accepts_ips_and_cidrs(): void
+    {
+        $this->assertTrue($this->passes('203.0.113.4'));
+        $this->assertTrue($this->passes('10.0.0.0/8'));
+        $this->assertTrue($this->passes('::1'));
+        $this->assertTrue($this->passes('2001:db8::/32'));
+    }
+
+    public function test_rejects_junk(): void
+    {
+        $this->assertFalse($this->passes('nope'));
+        $this->assertFalse($this->passes('10.0.0.0/99'));
+        $this->assertFalse($this->passes('1.2.3.4/'));
+    }
+}