You should try this:
class "pxe-clients-ia32" {
match if substring (option vendor-class-identifier, 0, 20)
= "PXEClient:Arch:00000";
filename "pxeboot_ia32_com0.bin";
}
and omit those lines to serve specific MAC addresses. Use this to serve the same boot loader to all clients that match the vendor-class-identifier.