Change your server configuration
The most direct solution is to install a network interface card (NIC) for each additional subnet. Then, configure each new NIC for the subnet it's connected to.
Use a bootpd relay
You can set up a bootpd relay to forward requests to another network subnet, without changing the network configuration. Read the bootpd man page for more details.
Change your network configuration
If you want NetBoot to work across subnets with only one NIC, set up a router or layer 3 switch. Configure the router or layer 3 switch between the NetBoot server and clients to let BootP data pass between the subnets.
If you use an external DHCP server, make sure that the broadcast data can pass between it and the NetBoot server.
Apple doesn't provide support to set up network hardware. To configure your router or switch, read the instructions from the hardware manufacturer.