diff --git a/zerver/lib/test_classes.py b/zerver/lib/test_classes.py
index 29c4a485d2..e05a203e4f 100644
--- a/zerver/lib/test_classes.py
+++ b/zerver/lib/test_classes.py
@@ -113,17 +113,10 @@ class ZulipTestCase(TestCase):
     def set_http_host(self, kwargs):
         # type: (Dict[str, Any]) -> None
         if 'subdomain' in kwargs:
-            if kwargs['subdomain'] != "":
-                kwargs["HTTP_HOST"] = "%s.%s" % (kwargs["subdomain"], settings.EXTERNAL_HOST)
-            else:
-                kwargs["HTTP_HOST"] = settings.EXTERNAL_HOST
+            kwargs['HTTP_HOST'] = Realm.host_for_subdomain(kwargs['subdomain'])
             del kwargs['subdomain']
         elif 'HTTP_HOST' not in kwargs:
-            if self.DEFAULT_SUBDOMAIN == "":
-                kwargs["HTTP_HOST"] = settings.EXTERNAL_HOST
-            else:
-                kwargs["HTTP_HOST"] = "%s.%s" % (self.DEFAULT_SUBDOMAIN,
-                                                 settings.EXTERNAL_HOST,)
+            kwargs['HTTP_HOST'] = Realm.host_for_subdomain(self.DEFAULT_SUBDOMAIN)
 
     @instrument_url
     def client_patch(self, url, info={}, **kwargs):
diff --git a/zerver/models.py b/zerver/models.py
index ace2f8849c..18a466f3c3 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -276,8 +276,13 @@ class Realm(ModelReprMixin, models.Model):
     @property
     def host(self):
         # type: () -> str
-        if self.subdomain not in [None, ""]:
-            return "%s.%s" % (self.subdomain, settings.EXTERNAL_HOST)
+        return self.host_for_subdomain(self.subdomain)
+
+    @staticmethod
+    def host_for_subdomain(subdomain):
+        # type: (str) -> str
+        if subdomain not in [None, ""]:
+            return "%s.%s" % (subdomain, settings.EXTERNAL_HOST)
         return settings.EXTERNAL_HOST
 
     @property