diff --git a/lib/compile/resolve.js b/lib/compile/resolve.js
index 7d06afa..cdd1a59 100644
--- a/lib/compile/resolve.js
+++ b/lib/compile/resolve.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var url = require('url')
+var URI = require('uri-js')
   , equal = require('fast-deep-equal')
   , util = require('./util')
   , SchemaObject = require('./schema_obj')
@@ -67,7 +67,7 @@ function resolve(compile, root, ref) {
  */
 function resolveSchema(root, ref) {
   /* jshint validthis: true */
-  var p = url.parse(ref, false, true)
+  var p = URI.parse(ref)
     , refPath = _getFullPath(p)
     , baseId = getFullPath(this._getId(root.schema));
   if (refPath !== baseId) {
@@ -115,9 +115,9 @@ var PREVENT_SCOPE_CHANGE = util.toHash(['properties', 'patternProperties', 'enum
 /* @this Ajv */
 function getJsonPointer(parsedRef, baseId, schema, root) {
   /* jshint validthis: true */
-  parsedRef.hash = parsedRef.hash || '';
-  if (parsedRef.hash.slice(0,2) != '#/') return;
-  var parts = parsedRef.hash.split('/');
+  parsedRef.fragment = parsedRef.fragment || '';
+  if (parsedRef.fragment.slice(0,1) != '/') return;
+  var parts = parsedRef.fragment.split('/');
 
   for (var i = 1; i < parts.length; i++) {
     var part = parts[i];
@@ -206,14 +206,13 @@ function countKeys(schema) {
 
 function getFullPath(id, normalize) {
   if (normalize !== false) id = normalizeId(id);
-  var p = url.parse(id, false, true);
+  var p = URI.parse(id);
   return _getFullPath(p);
 }
 
 
 function _getFullPath(p) {
-  var protocolSeparator = p.protocol || p.href.slice(0,2) == '//' ? '//' : '';
-  return (p.protocol||'') + protocolSeparator + (p.host||'') + (p.path||'')  + '#';
+  return URI.serialize(p).split('#')[0] + '#';
 }
 
 
@@ -225,7 +224,7 @@ function normalizeId(id) {
 
 function resolveUrl(baseId, id) {
   id = normalizeId(id);
-  return url.resolve(baseId, id);
+  return URI.resolve(baseId, id);
 }
 
 
@@ -246,7 +245,7 @@ function resolveIds(schema) {
       fullPath += '/' + (typeof keyIndex == 'number' ? keyIndex : util.escapeFragment(keyIndex));
 
     if (typeof id == 'string') {
-      id = baseId = normalizeId(baseId ? url.resolve(baseId, id) : id);
+      id = baseId = normalizeId(baseId ? URI.resolve(baseId, id) : id);
 
       var refVal = self._refs[id];
       if (typeof refVal == 'string') refVal = self._refs[refVal];
diff --git a/package.json b/package.json
index f5fdd57..f288812 100644
--- a/package.json
+++ b/package.json
@@ -62,7 +62,8 @@
   "dependencies": {
     "fast-deep-equal": "^1.0.0",
     "fast-json-stable-stringify": "^2.0.0",
-    "json-schema-traverse": "^0.3.0"
+    "json-schema-traverse": "^0.3.0",
+    "uri-js": "^3.0.2"
   },
   "devDependencies": {
     "ajv-async": "^1.0.0",
diff --git a/spec/resolve.spec.js b/spec/resolve.spec.js
index 1aa0017..f04eabe 100644
--- a/spec/resolve.spec.js
+++ b/spec/resolve.spec.js
@@ -91,6 +91,35 @@ describe('resolve', function () {
         });
       });
     });
+
+    it('should resolve ids defined as urn\'s (issue #423)', function() {
+      var schema = {
+        "type": "object",
+        "properties": {
+          "ip1": {
+            "$id": "urn:some:ip:prop",
+            "type": "string",
+            "format": "ipv4"
+          },
+          "ip2": {
+            "$ref": "urn:some:ip:prop"
+          }
+        },
+        "required": [
+          "ip1",
+          "ip2"
+        ]
+      };
+
+      var data = {
+        "ip1": "0.0.0.0",
+        "ip2": "0.0.0.0"
+      };
+      instances.forEach(function (ajv) {
+        var validate = ajv.compile(schema);
+        validate(data) .should.equal(true);
+      });
+    });
   });