First open system preferences and open Startup Disk.
If you see the Netboot server listed you can just select it and restart the machine.
If you don’t see the Netboot server listed you will need to tell your machine where the Netboot server is.
The proper way is have your network administrator enable IP helper for your Netboot server, but this is a quick an easy work around.
You can point your machine to startup from a specific Netboot server by using the following command;
sudo bless --netboot --server bsdp://10.70.0.25 --verbose
Just need to replace 10.70.0.25 with the IP address of your Netboot server. Now when you restart the machine it will boot from the Netboot server. This is very useful your clients and server are on different subnets.