Parent-Child Form Menus

I have two menu form elements that need to refelect a parent child relationship. There are 85 zones and 2200 zone-network combinations.

I want to bind the networkid element to an xml datasource that contains only the networks related to that zone. I have read about something like this before but have never implemented it.

create table tblZone ( zoneid int, zonename varchar(20))
create table tblNetwork ( networkid int, zoneid int, networkname varchar(20))

<select name="zoneid" onChange="reloadNetworks()">...</select>

<select name="networkid">...</select>