There are three options: Change your server hardware configuration (A), use a bootpd relay (B), or change your network hardware configuration (C).
 

A. Change server configuration

The most direct solution is to install a network interface card (NIC) for each additional subnet, then configure each new NIC properly for the subnet which it will be connected to.
 

B. Use a bootpd relay

You can set up a bootpd relay to forward requests to another network subnet without altering the network configuration. Refer to the bootpd man page for further details.
 

C. Change your network configuration

Note: Apple does not provide support for configuring network hardware. Refer to the hardware manufacturer for assistance with configuring your router or switch.

This is an advanced method that requires configuring a router or layer 3 switch. For NetBoot to work across subnets with only one NIC, the router or layer 3 switch between the NetBoot Server and clients must be configured to allow BootP data to pass between the subnets.

If you are using an external DHCP Server, make sure broadcast data can pass between the NetBoot Server and the external DHCP Server.