diff --git a/machines/stellio/configuration.nix b/machines/stellio/configuration.nix
index 0eb54e06..bc25f2a9 100644
--- a/machines/stellio/configuration.nix
+++ b/machines/stellio/configuration.nix
@@ -94,6 +94,20 @@
       22
     ];
 
+    # AIS delegates a single /64 over PPPoE (seen 2026-10-02: "ppp0: DHCP:
+    # received delegated prefix 2405:9800:b910:1f5d::/64"). The router
+    # service hands out sub-prefixes with SubnetId=auto, so whichever VLAN
+    # interface comes up first takes the whole /64 -- on this box vlan-mgmt
+    # did, and vlan-lan logged "Couldn't find a suitable prefix. Ran out of
+    # address space": no RA prefix on lan, so clients had no global IPv6.
+    # Pin the ids: lan (where the clients are) always gets subnet 0, i.e.
+    # the entire prefix when it is a /64; mgmt asks for subnet 1, which
+    # fails harmlessly on a /64 (mgmt is reached over v4 and the meshes) and
+    # starts working unchanged if AIS ever delegates wider. Drop once
+    # cnx-network's router service exposes a per-VLAN subnet id.
+    systemd.network.networks."40-vlan-lan".dhcpPrefixDelegationConfig.SubnetId = lib.mkForce "0x0";
+    systemd.network.networks."40-vlan-mgmt".dhcpPrefixDelegationConfig.SubnetId = lib.mkForce "0x1";
+
     systemd.services.heartbeat-push = {
       description = "Heartbeat push to uptime monitor";
       serviceConfig = {