add function to get IP representation of subnet mask

lib/ipv4.js

@@ -317,4 +317,15 @@ Address4.prototype.binaryZeroPad = function () {
   return padStart(this.bigInteger().toString(2), constants.BITS, '0');
 };
 
+/**
+ * Returns the ip representation of subnet mask
+ * @memberof Address4
+ * @instance
+ * @returns {string} subnetMaskIP
+ */
+Address4.prototype.subnetMaskIP = function () {
+  const shiftBits = constants.BITS - this.subnetMask;
+  return Address4.fromInteger(((0xffffffff << shiftBits) & 0xffffffff) >>> 0).correctForm()
+};
+
 module.exports = Address4;

package.json

@@ -7,7 +7,7 @@
     "browser",
     "validation"
   ],
-  "version": "6.2.0",
+  "version": "6.2.1",
   "author": "Beau Gunderson <[email protected]> (https://beaugunderson.com/)",
   "license": "MIT",
   "main": "ip-address.js",

test/functionality-v4-test.js

@@ -259,4 +259,12 @@ describe('v4', function () {
       });
     });
   });
+
+  describe('An IP with CIDR notation', function () {
+    var topic = new Address4('127.0.0.2/20');
+
+    it('should generate IP representation of subnet mask correctly', function () {
+      topic.subnetMaskIP().should.equal('255.255.240.0');
+    });
+  });
 });