For network booting in NAT mode, by default 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 modifyvm VM-name \
--nattftpserver1 10.0.2.2
$ VBoxManage modifyvm VM-name \
--nattftpfile1 /srv/tftp/boot/MyPXEBoot.pxe