diff --git a/oxi/interfaces/utils.py b/oxi/interfaces/utils.py
new file mode 100644
index 0000000..f91466d
--- /dev/null
+++ b/oxi/interfaces/utils.py
@@ -0,0 +1,25 @@
+def expand_vlan_range(value: str | list[str]) -> list[str]:
+    """Expand values like '1,7,14-15' into individual VLAN IDs."""
+    if isinstance(value, list):
+        value = ",".join(str(item) for item in value)
+
+    result: list[str] = []
+    if not value:
+        return result
+    for part in value.split(","):
+        part = part.strip()
+        if not part:
+            continue
+        if "-" in part:
+            start_s, end_s = part.split("-", 1)
+            try:
+                start, end = int(start_s), int(end_s)
+            except ValueError:
+                result.append(part)
+                continue
+            if start > end:
+                start, end = end, start
+            result.extend(str(i) for i in range(start, end + 1))
+        else:
+            result.append(part)
+    return result
diff --git a/tests/test_units.py b/tests/test_units.py
new file mode 100644
index 0000000..ad7cf32
--- /dev/null
+++ b/tests/test_units.py
@@ -0,0 +1,71 @@
+import pytest
+
+from conftest import load
+from oxi.exception import OxiAPIError
+from oxi.interfaces import device_registry
+from oxi.interfaces.base import BaseDevice
+from oxi.interfaces.contract import Interfaces, System
+from oxi.interfaces.utils import expand_vlan_range as eltex_expand
+from oxi.interfaces.utils import expand_vlan_range as qtech_expand
+
+
+class TestExpandVlanRange:
+    @pytest.mark.parametrize("expand", [qtech_expand, eltex_expand])
+    def test_simple_and_range(self, expand):
+        assert expand("1,7,14-15") == ["1", "7", "14", "15"]
+
+    @pytest.mark.parametrize("expand", [qtech_expand, eltex_expand])
+    def test_reversed_range_is_normalized(self, expand):
+        assert expand("15-13") == ["13", "14", "15"]
+
+    @pytest.mark.parametrize("expand", [qtech_expand, eltex_expand])
+    def test_non_numeric_range_kept_verbatim(self, expand):
+        assert expand("a-b") == ["a-b"]
+
+    @pytest.mark.parametrize("expand", [qtech_expand, eltex_expand])
+    def test_empty(self, expand):
+        assert expand("") == []
+
+    @pytest.mark.parametrize("expand", [qtech_expand, eltex_expand])
+    def test_list_input(self, expand):
+        assert expand(["1", "3-4"]) == ["1", "3", "4"]
+
+
+class TestKeeneticDecodeUtf:
+    @pytest.fixture(scope="class")
+    def keenetic(self):
+        return device_registry["keenetic"](load("keenetic"))
+
+    def test_plain_text_passthrough(self, keenetic):
+        assert keenetic._decode_utf("Plain ASCII") == "Plain ASCII"
+
+    def test_escaped_utf8_is_decoded(self, keenetic):
+        assert keenetic._decode_utf(r'"\xd0\x94\xd0\xbe\xd0\xbc"') == "Дом"
+
+
+class TestTemplateValidation:
+    def test_missing_required_group_raises(self):
+        class OnlySystem(BaseDevice):
+            template = "dummy.ttp"
+
+            def _load_template(self):
+                return '<group name="system"></group>'
+
+        with pytest.raises(ValueError, match="missing required groups"):
+            OnlySystem("data")
+
+    def test_missing_template_file_raises(self):
+        class NoTemplate(BaseDevice):
+            template = "does_not_exist.ttp"
+
+        with pytest.raises(FileNotFoundError):
+            NoTemplate("data")
+
+
+class TestNodeNotFound:
+    def test_not_found_config_raises_on_parse(self):
+        device = device_registry["eltex"](load("eltex", "not_found.conf"), name="HQ")
+        assert device.raw is None
+        with pytest.raises(OxiAPIError) as exc:
+            device.parse()
+        assert exc.value.status_code == 404