diff --git a/modules/crans/networking.nix b/modules/crans/networking.nix
index 37fcc4f..0cceae0 100644
--- a/modules/crans/networking.nix
+++ b/modules/crans/networking.nix
@@ -18,7 +18,7 @@ let
   isVm = cfg.id >= 100;
   isVmString = toString isVm;
   get_mac = id: vlan_num:
-    "02:00:00:0${toString (id >= 100)}:${toString (mod id 100)}:${toString vlan_num}";
+    "02:00:00:0${toString (id >= 100)}:${toString (mod id 100)}:${lib.fixedWidthString 2 "0" (toString vlan_num)}";
 in
 {
   options.crans.networking = {