diff --git a/lib/jobManager.js b/lib/jobManager.js
index 26ad9d7..48da15a 100644
--- a/lib/jobManager.js
+++ b/lib/jobManager.js
@@ -1,4 +1,5 @@
 var events = require('events');
+var crypto = require('crypto');
 
 var bignum = require('bignum');
 
@@ -14,18 +15,17 @@ var blockTemplate = require('./blockTemplate.js');
 
 
 //Unique extranonce per subscriber
-var ExtraNonceCounter = function(){
-    var instanceId = 31;
+var ExtraNonceCounter = function(configInstanceId){
+
+    var instanceId = configInstanceId || crypto.randomBytes(4).readUInt32LE(0);
     var counter = instanceId << 27;
-    var size = util.packUInt32BE(counter).length;
 
     this.next = function(){
-        var extraNonce = util.packUInt32BE(counter++);
+        var extraNonce = util.packUInt32BE(Math.abs(counter++));
         return extraNonce.toString('hex');
     };
-    this.size = function(){
-        return size;
-    };
+
+    this.size = 4; //bytes
 };
 
 //Unique job per new block template
@@ -126,7 +126,7 @@ var JobManager = module.exports = function JobManager(options){
     
     //public members
 
-    this.extraNonceCounter     = new ExtraNonceCounter();
+    this.extraNonceCounter     = new ExtraNonceCounter(options.instanceId);
     this.extraNoncePlaceholder = new Buffer('f000000ff111111f', 'hex');
     this.extraNonce2Size       = this.extraNoncePlaceholder.length - this.extraNonceCounter.size();