diff --git a/pyproject.toml b/pyproject.toml
index e6cc523e9..4236a4a11 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -119,6 +119,7 @@ markers = [
     "fortran: Fortran code",
     "integration: Full package build",
     "isolated: Needs an isolated virtualenv",
+    "network: Needs a network connection to setup or run",
     "setuptools: Tests setuptools integration",
     "virtualenv: Needs a virtualenv",
 ]
diff --git a/tests/conftest.py b/tests/conftest.py
index 48d7852d1..0fd0855e7 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -297,6 +297,7 @@ def pytest_collection_modifyitems(items: list[pytest.Item]) -> None:
         if "isolated" in getattr(item, "fixturenames", ()):
             item.add_marker(pytest.mark.virtualenv)
             item.add_marker(pytest.mark.isolated)
+            item.add_marker(pytest.mark.network)
 
 
 def pytest_report_header() -> str:
diff --git a/tests/test_fortran.py b/tests/test_fortran.py
index aee888256..598b0ce07 100644
--- a/tests/test_fortran.py
+++ b/tests/test_fortran.py
@@ -27,6 +27,7 @@
 @pytest.mark.compile()
 @pytest.mark.configure()
 @pytest.mark.fortran()
+@pytest.mark.network()
 @pytest.mark.skipif(shutil.which("gfortran") is None, reason="gfortran not available")
 @pytest.mark.skipif(
     sysconfig.get_platform().startswith("win"),
diff --git a/tests/test_pyproject_pep518.py b/tests/test_pyproject_pep518.py
index a3a4a5300..a46a98c82 100644
--- a/tests/test_pyproject_pep518.py
+++ b/tests/test_pyproject_pep518.py
@@ -10,7 +10,7 @@
 import pytest
 
 
-@pytest.mark.isolated()
+@pytest.mark.network()
 @pytest.mark.integration()
 def test_pep518_sdist(package_simple_pyproject_ext):
     correct_metadata = textwrap.dedent(