The developer manual (http://docs.nethserver.org/projects/nethserver-devel/en/latest/network.html) has a complete working example:
db networks set eth0 ethernet role green hwaddr xx:yy:27:DE:B6:51 ipaddr 192.168.1.4 netmask 255.255.255.0 network 192.168.1.0 onboot yes bootproto static
Usually, you only need to set ipaddr, netmask and gateway.