diff --git a/test_testping1.py b/test_testping1.py
index cc3f6a8..d6c0f73 100644
--- a/test_testping1.py
+++ b/test_testping1.py
@@ -27,6 +27,14 @@ def test_is_reachable_invalid_ip_format(self, mock_call):
         self.assertFalse(is_reachable('invalid_ip'))
         mock_call.assert_not_called()
 
+    @patch('testping1.subprocess.call')
+    def test_is_reachable_ip_too_long(self, mock_call):
+        """Test is_reachable rejects overly long IP strings to prevent DoS."""
+        with self.assertLogs(level='ERROR') as log:
+            self.assertFalse(is_reachable('A' * 101))
+            self.assertIn("IP address string too long", log.output[0])
+            mock_call.assert_not_called()
+
     @patch('testping1.subprocess.call')
     def test_is_reachable_argument_injection(self, mock_call):
         """Test is_reachable prevents argument injection by rejecting invalid IPs."""
diff --git a/testping1.py b/testping1.py
index 16af1e0..1c81739 100644
--- a/testping1.py
+++ b/testping1.py
@@ -15,6 +15,12 @@ def is_reachable(ip, timeout=1):
         bool: True if the ping is successful, False otherwise.
     """
 
+    # 🛡️ Sentinel: Add input length limit to prevent resource exhaustion (DoS)
+    # The ipaddress module can take significant time to parse extremely long strings
+    if isinstance(ip, str) and len(ip) > 100:
+        logging.error("IP address string too long")
+        return False
+
     # 🛡️ Sentinel: Validate IP address to prevent argument injection
     try:
         ip_obj = ipaddress.ip_address(ip)