Improve additional IP addresses validation

src/lib/y2network/dialogs/additional_address.rb

@@ -48,17 +48,28 @@ def contents
 
       def run
         ret = super
-        return ret if ret != :ok || @settings.subnet_prefix.start_with?("/")
+        return ret if ret != :ok
 
-        netmask = @settings.subnet_prefix
-        prefix = IPAddr.new("#{netmask}/#{netmask}").prefix
-        @settings.subnet_prefix = "/#{prefix}"
+        @settings.subnet_prefix = subnet_prefix_for?(@settings.subnet_prefix)
 
         ret
       end
 
     private
 
+      def subnet_prefix_for?(value)
+        return value if value.start_with?("/")
+
+        prefix =
+          if value.size < 3 || value =~ /^\d{3}$/
+            value.to_i
+          else
+            IPAddr.new("#{netmask}/#{netmask}").prefix
+          end
+
+        "/#{prefix}"
+      end
+
       def buttons
         [ok_button, cancel_button]
       end