For network booting in NAT mode, by default Oracle VM VirtualBox uses a built-in TFTP server at the IP address 10.0.2.4. This default behavior should work fine for typical remote-booting scenarios. However, it is possible to change the boot server IP and the location of the boot image with the following commands:
$ VBoxManage modifyvmVM-name \ --nattftpserver1 10.0.2.2 $ VBoxManage modifyvmVM-name \ --nattftpfile1 /srv/tftp/boot/MyPXEBoot.pxe