networks-for-postcode.jq

foreach inputs as $in (
  {};
  if has("postcode") and has("network") then null
  else . as $x
  | $in
  | if length == 1 and .[0][-1] == "Record" then {}
    elif length == 2 and .[0][-1] == "Network" then ($x + {network: .[-1]})
    elif length == 2 and .[0][-2] == "postal" and .[0][-1] == "code" and .[-1] == "12061" then ($x + {postcode: .[-1]})
    else $x
    end
  end;
  select( has("postcode") and has("network") ) | .network
)

Run as: mmdbinspect -db /usr/local/share/maxmind/GeoIP/GeoIP2-City.mmdb 0.0.0.0/0 | jq --stream -nr -f networks-for-postcode.jq.

Note that mmdbinspect is not fully capable of being used like this as it doesn't stream its output. Memory runs out.