diff --git a/py2http/examples/object_dispatch.py b/py2http/examples/object_dispatch.py
index 77230da..d7eba36 100644
--- a/py2http/examples/object_dispatch.py
+++ b/py2http/examples/object_dispatch.py
@@ -11,7 +11,7 @@
 
 from typing import MutableMapping
 
-from py2http.service import shared_objects
+from py2http.service import run_app, shared_objects
 
 
 class BackendStore(MutableMapping):
@@ -60,7 +60,7 @@ def __delitem__(self, key: str):
 app = mk_app(handlers, publish_openapi=True, publish_swagger=True)
 
 
-def test_service(rooturl="http://localhost:8080"):
+def test_service(rooturl="http://localhost:3030"):
     import requests
 
     url = f"{rooturl}/backend_store"
@@ -75,7 +75,9 @@ def test_service(rooturl="http://localhost:8080"):
 
 
 if __name__ == "__main__":
-    app.run()
+    # Not `app.run()`: that is bottle's own runner (port 8080), while the OpenAPI spec (and
+    # so Swagger's "Try it out") points at the port `run_app` serves on, 3030 by default.
+    run_app(app)
 
 
 # old code (with errors)
diff --git a/py2http/openapi_utils.py b/py2http/openapi_utils.py
index ea6127d..0ddc9de 100644
--- a/py2http/openapi_utils.py
+++ b/py2http/openapi_utils.py
@@ -34,8 +34,12 @@ def openapi_type_mapping(obj_type):
         return oatype_for_pytype.get(obj_type, None)
 
 
+from py2http.default_configs import default_configs
+
 BINARY = "binary"
-DFLT_SERVER_URL = "http://localhost:3030"
+# Where `run_app` serves unless told otherwise: the spec's server URL must match it, or
+# Swagger's "Try it out" calls a port nothing listens on (i2mint/py2http#13).
+DFLT_SERVER_URL = f"http://{default_configs['host']}:{default_configs['port']}"
 
 
 class OpenApiExtractor:
diff --git a/py2http/tests/test_example_ports.py b/py2http/tests/test_example_ports.py
new file mode 100644
index 0000000..e0d2df6
--- /dev/null
+++ b/py2http/tests/test_example_ports.py
@@ -0,0 +1,15 @@
+"""The example services must listen where their OpenAPI spec says (i2mint/py2http#13)."""
+
+from py2http.default_configs import default_configs
+from py2http.openapi_utils import DFLT_SERVER_URL, mk_openapi_template
+
+
+def test_openapi_server_url_is_where_run_app_serves():
+    assert DFLT_SERVER_URL == f"http://{default_configs['host']}:{default_configs['port']}"
+    assert mk_openapi_template()["servers"] == [{"url": DFLT_SERVER_URL}]
+
+
+def test_object_dispatch_example_checks_the_port_the_spec_names():
+    from py2http.examples import object_dispatch as ex
+
+    assert ex.test_service.__defaults__ == (DFLT_SERVER_URL,)