diff --git a/src/registrar/tests/common.py b/src/registrar/tests/common.py
index 7a0d2ec59..5a3887f3b 100644
--- a/src/registrar/tests/common.py
+++ b/src/registrar/tests/common.py
@@ -634,6 +634,15 @@ class MockEppLib(TestCase):
         ],
         addrs=["1.2.3.4", "2.3.4.5"],
     )
+    infoDomainCheckHostIPCombo = fakedEppObject(
+        "nameserversubdomain.gov",
+        cr_date=datetime.datetime(2023, 5, 25, 19, 45, 35),
+        contacts=[],
+        hosts=[
+            "ns1.nameserversubdomain.gov",
+            "ns2.nameserversubdomain.gov",
+        ],
+    )
 
     # TODO-848: Fix naming
     extendedValues = False
@@ -650,6 +659,8 @@ class MockEppLib(TestCase):
                 return MagicMock(res_data=[self.infoDomainNoHost])
         elif getattr(_request, "name", None) == "nameserverwithip.gov":
             return MagicMock(res_data=[self.infoDomainHasIP])
+        elif getattr(_request, "name", None) == "namerserversubdomain.gov":
+            return MagicMock(res_data=[self.infoDomainCheckHostIPCombo])
         # elif getattr(_request, "name", None) == "failednameserver.gov":
         #     return MagicMock(res_data=[self.infoDomainUpdateFail])
         return MagicMock(res_data=[self.mockDataInfoDomain])
diff --git a/src/registrar/tests/test_models_domain.py b/src/registrar/tests/test_models_domain.py
index 701f9e081..c0eac751d 100644
--- a/src/registrar/tests/test_models_domain.py
+++ b/src/registrar/tests/test_models_domain.py
@@ -1134,6 +1134,40 @@ class TestRegistrantNameservers(MockEppLib):
         self.mockedSendFunction.assert_has_calls(expectedCalls, any_order=True)
         self.assertEqual(self.mockedSendFunction.call_count, 4)
 
+    def test_is_subdomain_with_no_ip(self):
+        domain, _ = Domain.objects.get_or_create(
+            name="nameserversubdomain.gov", state=Domain.State.READY
+        )
+
+        with self.assertRaises(ValueError):
+            domain.nameservers = [
+                ("ns1.nameserversubdomain.gov",),
+                ("ns2.nameserversubdomain.gov",),
+            ]
+
+    @skip("not implemented yet")
+    def test_not_subdomain_but_has_ip(self):
+        # TO FIX - Logic
+        # domain, _ = Domain.objects.get_or_create(
+        #     name="nameserversubdomain.gov", state=Domain.State.READY
+        # )
+
+        # with self.assertRaises(ValueError):
+        #     domain.nameservers = [("ns1.cats-da-best.gov", ["1.2.3.4"]),
+        #     ("ns2.cats-da-best.gov", ["2.3.4.5"]),]
+        raise
+
+    def test_is_subdomain_but_ip_addr_not_valid(self):
+        domain, _ = Domain.objects.get_or_create(
+            name="nameserversubdomain.gov", state=Domain.State.READY
+        )
+
+        with self.assertRaises(ValueError):
+            domain.nameservers = [
+                ("ns1.nameserversubdomain.gov", ["1.2.3"]),
+                ("ns2.nameserversubdomain.gov", ["2.3.4"]),
+            ]
+
     @skip("not implemented yet")
     def test_caching_issue(self):
         raise