In some cases it may be useful to add non visible in Full BGP table prefixes for better quality of reports.
Or it may be used to override inaccurate data.
Starting from FastNetMon 2.0.351 we use JSON format for these files and it can be easily altered:
$ head -n 1 /var/cache/fastnetmon/prefix_asn_mapping_ipv4.json
$ head -n 1 /var/cache/fastnetmon/prefix_asn_mapping_ipv6.json