diff --git a/zerver/lib/subdomains.py b/zerver/lib/subdomains.py
index 4b74a9c7a7..e05fa2d64f 100644
--- a/zerver/lib/subdomains.py
+++ b/zerver/lib/subdomains.py
@@ -17,10 +17,10 @@ def get_subdomain(request):
     # X-Forwarded-Host, it passes right through the same way.  So our
     # logic is a bit complicated to allow for that variation.
     #
-    # For EXTERNAL_HOST, we take a missing port to mean that any port
-    # should be accepted in Host.  It's not totally clear that's the
-    # right behavior, but it keeps compatibility with older versions
-    # of Zulip, so that's a start.
+    # For both EXTERNAL_HOST and REALM_HOSTS, we take a missing port
+    # to mean that any port should be accepted in Host.  It's not
+    # totally clear that's the right behavior, but it keeps
+    # compatibility with older versions of Zulip, so that's a start.
 
     host = request.get_host().lower()
 
@@ -32,6 +32,11 @@ def get_subdomain(request):
             return Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
         return subdomain
 
+    for subdomain, realm_host in settings.REALM_HOSTS.items():
+        if re.search('^%s(:\d+)?$' % (realm_host,),
+                     host):
+            return subdomain
+
     return Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
 
 def is_subdomain_root_or_alias(request):
diff --git a/zerver/models.py b/zerver/models.py
index f6d477c4a2..b257706ee7 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -280,9 +280,10 @@ class Realm(models.Model):
     @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
+        if subdomain == Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
+            return settings.EXTERNAL_HOST
+        default_host = "%s.%s" % (subdomain, settings.EXTERNAL_HOST)
+        return settings.REALM_HOSTS.get(subdomain, default_host)
 
     @property
     def is_zephyr_mirror_realm(self):
diff --git a/zerver/tests/test_subdomains.py b/zerver/tests/test_subdomains.py
index cbdb283f30..f0c273027f 100644
--- a/zerver/tests/test_subdomains.py
+++ b/zerver/tests/test_subdomains.py
@@ -1,6 +1,6 @@
 
 import mock
-from typing import Any, List
+from typing import Any, Dict, List
 
 from django.test import TestCase, override_settings
 
@@ -18,9 +18,10 @@ class SubdomainsTest(TestCase):
             return request
 
         def test(expected, host, *, plusport=True,
-                 external_host='example.org', root_aliases=[]):
-            # type: (str, str, bool, str, List[str]) -> None
+                 external_host='example.org', realm_hosts={}, root_aliases=[]):
+            # type: (str, str, bool, str, Dict[str, str], List[str]) -> None
             with self.settings(EXTERNAL_HOST=external_host,
+                               REALM_HOSTS=realm_hosts,
                                ROOT_SUBDOMAIN_ALIASES=root_aliases):
                 self.assertEqual(get_subdomain(request_mock(host)), expected)
                 if plusport and ':' not in host:
@@ -38,11 +39,18 @@ class SubdomainsTest(TestCase):
         test(ROOT, 'arbitrary.com')
         test(ROOT, 'foo.example.org.evil.com')
 
-        # Any port is fine in Host if there's none in EXTERNAL_HOST
+        # REALM_HOSTS adds a name,
+        test('bar', 'chat.barbar.com', realm_hosts={'bar': 'chat.barbar.com'})
+        # ... exactly, ...
+        test(ROOT, 'surchat.barbar.com', realm_hosts={'bar': 'chat.barbar.com'})
+        test(ROOT, 'foo.chat.barbar.com', realm_hosts={'bar': 'chat.barbar.com'})
+        # ... and leaves the subdomain in place too.
+        test('bar', 'bar.example.org', realm_hosts={'bar': 'chat.barbar.com'})
+
+        # Any port is fine in Host if there's none in EXTERNAL_HOST, ...
         test('foo', 'foo.example.org:443', external_host='example.org')
         test('foo', 'foo.example.org:12345', external_host='example.org')
-
-        # Explicit port in EXTERNAL_HOST must be explicitly matched in Host
+        # ... but an explicit port in EXTERNAL_HOST must be explicitly matched in Host.
         test(ROOT, 'foo.example.org', external_host='example.org:12345')
         test(ROOT, 'foo.example.org', external_host='example.org:443', plusport=False)
         test('foo', 'foo.example.org:443', external_host='example.org:443')
diff --git a/zproject/settings.py b/zproject/settings.py
index d67c722662..065f6d1aab 100644
--- a/zproject/settings.py
+++ b/zproject/settings.py
@@ -245,6 +245,12 @@ DEFAULT_SETTINGS.update({
     # purpose now that the REALMS_HAVE_SUBDOMAINS migration is finished.
     'SYSTEM_ONLY_REALMS': {"zulip"},
 
+    # Alternate hostnames to serve particular realms on, in addition to
+    # their usual subdomains.  Keys are realm string_ids (aka subdomains),
+    # and values are alternate hosts.
+    # The values will also be added to ALLOWED_HOSTS.
+    'REALM_HOSTS': {},
+
     # Whether the server is using the Pgroonga full-text search
     # backend.  Plan is to turn this on for everyone after further
     # testing.
@@ -411,9 +417,11 @@ USE_X_FORWARDED_HOST = True
 
 # Extend ALLOWED_HOSTS with localhost (needed to RPC to Tornado),
 ALLOWED_HOSTS += ['127.0.0.1', 'localhost']
-# and with hosts corresponding to EXTERNAL_HOST.
+# ... with hosts corresponding to EXTERNAL_HOST,
 ALLOWED_HOSTS += [EXTERNAL_HOST.split(":")[0],
                   '.' + EXTERNAL_HOST.split(":")[0]]
+# ... and with the hosts in REALM_HOSTS.
+ALLOWED_HOSTS += REALM_HOSTS.values()
 
 MIDDLEWARE = (
     # With the exception of it's dependencies,