Hi @george1421 :
Sorry for reply you late.

There was two things that I question.

The router for your network is configured as the FOG server at 10.0.2.15

I have check it again. My network is configured to the FOG at 10.0.2.15.

The subnet mask for your network is 255.0.0.0 ?? is that true?

I also check that the subnet is 255.0.0.0 .

And I found that it works when I change my dhcp server from dnsmasq to isc-dhcp-server by following your link.

Thank you very much @george1421 for your help.