diff --git a/lib/build-vms.nix b/lib/build-vms.nix
index efd1ce3b8d3..2c867a0985b 100644
--- a/lib/build-vms.nix
+++ b/lib/build-vms.nix
@@ -108,7 +108,7 @@ rec {
                   
                   virtualisation.qemu.options =
                     lib.flip lib.concatMapStrings interfacesNumbered
-                      ({ first, second }: qemuNICFlags second first );
+                      ({ first, second }: qemuNICFlags second first m.second);
                 };
             }
           )
diff --git a/lib/qemu-flags.nix b/lib/qemu-flags.nix
index 05edc1d2cc6..7831c8f149a 100644
--- a/lib/qemu-flags.nix
+++ b/lib/qemu-flags.nix
@@ -2,8 +2,8 @@
 
 {
 
-  qemuNICFlags = nic: net:
-    "-net nic,vlan=${toString nic},model=virtio " +
+  qemuNICFlags = nic: net: machine:
+    "-net nic,vlan=${toString nic},macaddr=52:54:00:12:${toString net}:${toString machine},model=virtio " +
     # Use 232.0.1.<vlan> as the multicast address to connect VMs on
     # the same vlan, but allow it to be overriden using the
     # $QEMU_MCAST_ADDR_<vlan> environment variable.  The test driver