IP addresses print CIDR of /? if it is not a real CIDR approved subnet mask. For example, a subnet mask of .250 gives a /?, but .252 gives /30. While this is not real-life (real life does not tell us when we use a foolish subnet), it helps considerably when testing things.

EduNetworkBuilder/IPAddress.cs

@@ -6,7 +6,7 @@ using System.Threading.Tasks;
 using System.Globalization;
 using System.Xml;
 using System.Windows.Forms;
-
+using System.Text.RegularExpressions;
 
 namespace EduNetworkBuilder
 {
@@ -140,19 +140,24 @@ namespace EduNetworkBuilder
                 }
             }
             else
+            {                
+                _mask = MaskNumFromCIDRString(mySplitVal[1]);
+            }
+        }
+
+        static UInt32 MaskNumFromCIDRString(string cidr)
         {
             UInt32 tInt = 0;
             int cdr;
-                int.TryParse(mySplitVal[1], out cdr);
+            int.TryParse(cidr, out cdr);
             for (int loop = 0; loop < 32; loop++)
             {
                 tInt = (tInt << 1);
                 if (loop < cdr) tInt++;
             }
-                _mask = tInt;
-            }
-        }
             
+            return tInt;
+        }
 
         public bool Equals(UInt32 IP)
         {
@@ -228,6 +233,31 @@ namespace EduNetworkBuilder
                 PadIt(_ip.ToIpString()), PadIt(_mask.ToIpString()), PadIt(gw));
             return tstring;
         }
+
+        /// <summary>
+        /// Return the CIDR number for this address.  This is the number of 1s before the first 0
+        /// </summary>
+        /// <returns></returns>
+        public int CIDRNumber()
+        {
+            string mask = GetMask.ToBitString();
+            mask = Regex.Replace(mask, "0", "");
+            return mask.Length;
+        }
+
+        /// <summary>
+        /// Return true if the subnet mask is really a true CIDR string /8, /16, etc.
+        /// It is false if the subnet mask creates a mask that does not map to CIDR.  For
+        /// example, 255.255.255.250 is not a real subnet mask.  252 is.
+        /// </summary>
+        /// <returns>True if there are no 1s after the first 0</returns>
+        public bool ValidCIDR()
+        {
+            int cidr = CIDRNumber();
+            UInt32 tMask = MaskNumFromCIDRString(cidr.ToString());
+            if (tMask == _mask) return true;
+            return false;
+        }
     }
 
     public static class IpHelpers

EduNetworkBuilder/NetworkInterface.cs

@@ -151,10 +151,10 @@ namespace EduNetworkBuilder
             }
             else
             {
-                string mask = myIP.GetMask.ToBitString();
+                int cidr = myIP.CIDRNumber();
-                mask = Regex.Replace(mask, "0", "");
+                string answer = cidr.ToString();
-                int count = mask.Length;
+                if (!myIP.ValidCIDR()) answer = "?";
-                tstring += "/" + count.ToString();
+                tstring += "/" + answer;
             }
             return tstring;
         }