diff --git a/lib/cmdlib.py b/lib/cmdlib.py
index 40dca63f54817c0140816760b9cef1bab7264afd..a3d9fc9d82681c82ec0a8a99aa02a7717876e900 100644
--- a/lib/cmdlib.py
+++ b/lib/cmdlib.py
@@ -8002,6 +8002,11 @@ def _RemoveDisks(lu, instance, target_node=None):
                       " continuing anyway: %s", device.iv_name, node, msg)
         all_result = False
 
+    # if this is a DRBD disk, return its port to the pool
+    if device.dev_type in constants.LDS_DRBD:
+      tcp_port = device.logical_id[2]
+      lu.cfg.AddTcpUdpPort(tcp_port)
+
   if instance.disk_template == constants.DT_FILE:
     file_storage_dir = os.path.dirname(instance.disks[0].logical_id[1])
     if target_node:
@@ -11161,6 +11166,11 @@ class LUInstanceSetParams(LogicalUnit):
         self.LogWarning("Could not remove metadata for disk %d on node %s,"
                         " continuing anyway: %s", idx, pnode, msg)
 
+    # this is a DRBD disk, return its port to the pool
+    for disk in old_disks:
+      tcp_port = disk.logical_id[2]
+      self.cfg.AddTcpUdpPort(tcp_port)
+
   def Exec(self, feedback_fn):
     """Modifies an instance.
 
@@ -11187,6 +11197,11 @@ class LUInstanceSetParams(LogicalUnit):
             self.LogWarning("Could not remove disk/%d on node %s: %s,"
                             " continuing anyway", device_idx, node, msg)
         result.append(("disk/%d" % device_idx, "remove"))
+
+        # if this is a DRBD disk, return its port to the pool
+        if device.dev_type in constants.LDS_DRBD:
+          tcp_port = device.logical_id[2]
+          self.cfg.AddTcpUdpPort(tcp_port)
       elif disk_op == constants.DDM_ADD:
         # add a new disk
         if instance.disk_template in (constants.DT_FILE,
diff --git a/lib/config.py b/lib/config.py
index fdd77c0d918dcf43ae3628c1326376b5380d5742..7a8faa19a36cb9bb5caa79b6e3ebcacb3882a089 100644
--- a/lib/config.py
+++ b/lib/config.py
@@ -1158,6 +1158,14 @@ class ConfigWriter:
     """
     if instance_name not in self._config_data.instances:
       raise errors.ConfigurationError("Unknown instance '%s'" % instance_name)
+
+    # If a network port has been allocated to the instance,
+    # return it to the pool of free ports.
+    inst = self._config_data.instances[instance_name]
+    network_port = getattr(inst, "network_port", None)
+    if network_port is not None:
+      self._config_data.cluster.tcpudp_port_pool.add(network_port)
+
     del self._config_data.instances[instance_name]
     self._config_data.cluster.serial_no += 1
     self._WriteConfig()