{% macro net_param(param, value) -%}
  {% if value -%}
  {{ param }} {{ value }}
  {%- endif %}
{%- endmacro %}

auto {{ net_if }}
iface {{ net_if }} inet static
  address {{ net_ip }}
  netmask {{ net_mask }}
  network {{ net_addr }}
  {{ net_param('gateway', net_gw) }}
  {{ net_param('broadcast', net_bcast) }}
# end