if you do not have ipv6, look at https://lab.nexedi.com/yusei/a/blob/master/slapos.cfg (slapformat section)
input_definition_file = /etc/opt/slapos/computer_definition.cfg
https://lab.nexedi.com/yusei/a/blob/master/computer_definition.cfg
I don't use this option recently, but it should work.