diff --git a/compose/project.py b/compose/project.py
index bc093628c..6446a6d33 100644
--- a/compose/project.py
+++ b/compose/project.py
@@ -178,7 +178,7 @@ class Project(object):
             del service_dict['net']
 
         else:
-            net = 'bridge'
+            net = None
 
         return net
 
diff --git a/compose/service.py b/compose/service.py
index 1e91a9f23..12a021bfb 100644
--- a/compose/service.py
+++ b/compose/service.py
@@ -473,7 +473,7 @@ class Service(object):
 
     def _get_net(self):
         if not self.net:
-            return "bridge"
+            return None
 
         if isinstance(self.net, Service):
             containers = self.net.containers()
diff --git a/tests/unit/project_test.py b/tests/unit/project_test.py
index fc49e9b88..9ee6f28c3 100644
--- a/tests/unit/project_test.py
+++ b/tests/unit/project_test.py
@@ -209,6 +209,18 @@ class ProjectTest(unittest.TestCase):
         ], None)
         self.assertEqual(project.get_service('test')._get_volumes_from(), container_ids)
 
+    def test_net_unset(self):
+        mock_client = mock.create_autospec(docker.Client)
+        project = Project.from_dicts('test', [
+            {
+                'name': 'test',
+                'image': 'busybox:latest',
+            }
+        ], mock_client)
+        service = project.get_service('test')
+        self.assertEqual(service._get_net(), None)
+        self.assertNotIn('NetworkMode', service._get_container_host_config({}))
+
     def test_use_net_from_container(self):
         container_id = 'aabbccddee'
         container_dict = dict(Name='aaa', Id=container_id)