By default, FastNetMon allocates memory for all networks in networks_list. With flexible counters option it will allocate memory only for really used hosts with traffic.
Update! In FastNetMon 2.0.166 we’ve enabled flexible counters by default for all new installations.
You can easily add really big networks like /8 or even bigger and FastNetMon will allocate memory on demand.
You can enable this feature this way:
sudo fcli set main flexible_traffic_calculation enable
For debugging purposes, you can check number of counters allocated for hosts in your network this way:
sudo fcli show system_counters|grep hosts_hash_size