diff --git a/synapse/config/server.py b/synapse/config/server.py
index e15bceb2968..e829430be75 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -169,6 +169,8 @@ def generate_ip_set(
     "fc00::/7",
     # Testing networks.
     "2001:db8::/32",
+    # RFC9637: second IPv6 documentation prefix
+    "3fff::/20",
     # Multicast.
     "ff00::/8",
     # Site-local addresses