diff --git a/EduNetworkBuilder/NetworkInterface.cs b/EduNetworkBuilder/NetworkInterface.cs
index 2b715ee..99ab2ed 100644
--- a/EduNetworkBuilder/NetworkInterface.cs
+++ b/EduNetworkBuilder/NetworkInterface.cs
@@ -123,6 +123,16 @@ namespace EduNetworkBuilder
             return VLANTagType.Forbidden; //if not defined, it is forbidden
         }
 
+        public int GetUntaggedVLAN()
+        {
+            foreach (VLANInfo vin in VLANs)
+            {
+                if (vin.Tag == VLANTagType.Untagged)
+                    return vin.ID;
+            }
+            return NB.InvalidVLAN;
+        }
+
         public int GetNonForbiddenVLANID()
         {
             foreach(VLANInfo vin in VLANs)
@@ -130,7 +140,7 @@ namespace EduNetworkBuilder
                 if (vin.Tag != VLANTagType.Forbidden)
                     return vin.ID;
             }
-            return -999;
+            return NB.InvalidVLAN;
         }
 
         public void SetVLANTag(int id, VLANTagType Tag)