diff --git a/public/api.php b/public/api.php
index 03fb1e8..4f6cbb9 100644
--- a/public/api.php
+++ b/public/api.php
@@ -2798,7 +2798,7 @@ function lan_client_rows(array $leases, array $aliases, array $wifiMacs = [], st
             'connected_time' => $isFdbVisible ? 'N/A' : '-',
             'inactive_time' => '-',
             'presence' => $isFdbVisible ? 'fdb' : 'lease',
-            'wol_available' => true,
+            'wol_available' => !$isFdbVisible,
             'rx_bytes' => 0,
             'tx_bytes' => 0,
             'rx_packets' => 0,
@@ -2822,7 +2822,12 @@ function send_wake_on_lan(string $mac, string $broadcast = '192.168.50.255', int
         throw new InvalidArgumentException('unknown_dhcp_client');
     }
 
-    $lanMacs = array_fill_keys(array_merge(lan_fdb_macs(), observed_lan_macs($leases)), true);
+    $activeLanMacs = array_fill_keys(lan_fdb_macs(), true);
+    if (isset($activeLanMacs[$mac])) {
+        throw new InvalidArgumentException('device_already_online');
+    }
+
+    $lanMacs = array_fill_keys(array_merge(array_keys($activeLanMacs), observed_lan_macs($leases)), true);
     if (!isset($lanMacs[$mac])) {
         throw new InvalidArgumentException('not_lan_client');
     }
diff --git a/public/assets/app.js b/public/assets/app.js
index 594f6d3..0f59ec6 100644
--- a/public/assets/app.js
+++ b/public/assets/app.js
@@ -187,6 +187,7 @@
             wolConfirm: ({ hostname, mac }) => `Send one Wake-on-LAN request to ${hostname || mac}?`,
             wolSent: 'Wake-on-LAN request sent.',
             wolFailed: 'Wake-on-LAN request failed',
+            wolAlreadyOnline: 'LAN device is already online.',
             show: 'Show',
             hide: 'Hide',
             translateButton: 'Translate',
@@ -338,6 +339,7 @@
             wolConfirm: ({ hostname, mac }) => `${hostname || mac} 기기에 WOL 요청을 1회 보낼까요?`,
             wolSent: 'WOL 요청을 보냈습니다.',
             wolFailed: 'WOL 요청 실패',
+            wolAlreadyOnline: 'LAN 기기가 이미 켜져 있습니다.',
             show: '보기',
             hide: '숨기기',
             translateButton: '번역',
@@ -603,6 +605,13 @@
         message,
     }, options));
 
+    function apiErrorMessage(message) {
+        const map = {
+            device_already_online: t('wolAlreadyOnline'),
+        };
+        return map[message] || message;
+    }
+
     async function api(action, body = null) {
         const opts = {
             method: body ? 'POST' : 'GET',
@@ -627,7 +636,7 @@
         const res = await fetch(url, opts);
         const json = await res.json();
         if (!res.ok || !json.ok) {
-            throw new Error(json?.message || json?.error || 'API error');
+            throw new Error(apiErrorMessage(json?.message || json?.error || 'API error'));
         }
         return json.data;
     }
diff --git a/public/index.php b/public/index.php
index b955831..e91701b 100644
--- a/public/index.php
+++ b/public/index.php
@@ -363,7 +363,7 @@ html[data-theme="light"] .details-panel{background:#edf3fa;border-color:#b8c7da}
         </div>
     </div>
 </div>
-<script src="/assets/app.js?v=20260721_sensor_charts1"></script>
+<script src="/assets/app.js?v=20260721_lan_wol_guard1"></script>
 <script src="/assets/wakelock.js?v=20260607_prefs1"></script>
 <?php endif; ?>
 </body>