We are currently Netbooting across several subnets with servers that are sitting in a central location. If you are using Cisco equipment for your networking equipment, the command to setup a helper address is very simple. I am not sure if you are the one who deals with that piece. The command on the router to enable the helper address is ip helper-address x.x.x.x. Just change the x's with the IP address of your Netboot server or DHCP server. You can have more than one helper address per VLAN on the router. I would guess that there are already helper addresses in place on your network unless you have Netboot servers and DHCP servers at each of your sites.