diff --git a/group_vars/all/vars.yml b/group_vars/all/vars.yml
index c3f0502..cc30765 100644
--- a/group_vars/all/vars.yml
+++ b/group_vars/all/vars.yml
@@ -78,3 +78,9 @@ radius_pg_re2o_ro_password: "{{ vault_re2o_db_user_passwords.re2o_ro }}"
 
 
 apartment_block_dhcp: "{{ apartment_block }}"
+
+
+
+# Careful, this is not byte-aligned, just nibble-aligned (RIPE gave us a /28).
+# However, we ALWAYS keep the trailing 0 to have byte alignment.
+ipv6_base_prefix: "2a09:6840"
diff --git a/roles/radvd/templates/radvd.conf.j2 b/roles/radvd/templates/radvd.conf.j2
index f773514..bf301a9 100644
--- a/roles/radvd/templates/radvd.conf.j2
+++ b/roles/radvd/templates/radvd.conf.j2
@@ -14,7 +14,7 @@
 #     MaxRtrAdvInterval 30;
 # 
 # 
-#     prefix 2a09:6840:{{ subnet_ids.ap }}::/64  {
+#     prefix {{ ipv6_base_prefix }}:{{ subnet_ids.ap }}::/64  {
 #         AdvRouterAddr on;
 #     };
 # 
@@ -22,8 +22,8 @@
 #     DNSSL borne.auro.re {};
 # 
 #     # Les DNS récursifs
-#     RDNSS 2a09:6840:{{ subnet_ids.ap }}::{{ dns_host_suffix_main }} {};
-#     RDNSS 2a09:6840:{{ subnet_ids.ap }}::{{ dns_host_suffix_backup }} {};
+#     RDNSS {{ ipv6_base_prefix }}:{{ subnet_ids.ap }}::{{ dns_host_suffix_main }} {};
+#     RDNSS {{ ipv6_base_prefix }}:{{ subnet_ids.ap }}::{{ dns_host_suffix_backup }} {};
 # };
 
 
@@ -36,14 +36,14 @@ interface ens20 {
     AdvDefaultPreference high;
     MaxRtrAdvInterval 30;
 
-    prefix 2a09:6840:{{ subnet_ids.users_wired }}::/64  {
+    prefix {{ ipv6_base_prefix }}:{{ subnet_ids.users_wired }}::/64  {
         AdvRouterAddr on;
     };
 
     DNSSL fil.{{ apartment_block_dhcp }}.auro.re {}; # TODO: fix this shitty workaround.
 
-    RDNSS 2a09:6840:{{ subnet_ids.users_wired }}::{{ dns_host_suffix_main }} {};
-    RDNSS 2a09:6840:{{ subnet_ids.users_wired }}::{{ dns_host_suffix_backup }} {};
+    RDNSS {{ ipv6_base_prefix }}:{{ subnet_ids.users_wired }}::{{ dns_host_suffix_main }} {};
+    RDNSS {{ ipv6_base_prefix }}:{{ subnet_ids.users_wired }}::{{ dns_host_suffix_backup }} {};
 };
 
 
@@ -56,14 +56,14 @@ interface ens21 {
     AdvDefaultPreference high;
     MaxRtrAdvInterval 30;
 
-    prefix 2a09:6840:{{ subnet_ids.users_wifi }}::/64  {
+    prefix {{ ipv6_base_prefix }}:{{ subnet_ids.users_wifi }}::/64  {
         AdvRouterAddr on;
     };
 
     DNSSL wifi.{{ apartment_block_dhcp }}.auro.re {}; # TODO: fix this shitty workaround.
 
-    RDNSS 2a09:6840:{{ subnet_ids.users_wifi }}::{{ dns_host_suffix_main }} {};
-    RDNSS 2a09:6840:{{ subnet_ids.users_wifi }}::{{ dns_host_suffix_backup }} {};
+    RDNSS {{ ipv6_base_prefix }}:{{ subnet_ids.users_wifi }}::{{ dns_host_suffix_main }} {};
+    RDNSS {{ ipv6_base_prefix }}:{{ subnet_ids.users_wifi }}::{{ dns_host_suffix_backup }} {};
 };