Javascript Zip Code Lookup For City And State?

I'm trying to write a Javascript Zip Code lookup utility.

1) The User will select a State from a drop down.

2) The City Drown Down will then be populated based on the State selected

3) A textbox below is then populated with the corresponding Zip Code(s) based on Selected State and Zip Code on an ASP.NET page.

Problem: I get the error Request() is not defined which causes the application to hault when a State is selected.

What should I cahnge in the following code?

<script language="javascript">
var request = new Request();
var url = "";
function getZipCode()
{
request.GetNoCache(url + "getZipCode.aspx?City=" +document.getElementById("drpCity").options[document.getElementById("drpCity").selectedIndex].value+ "&State=" +document.getElementById("drpStateSearch").options[document.getElementById("drpStateSearch").selectedIndex].value,
function(result)........