This directive is available to provide privacy for users. When Squid writes access.log and other log files, it applies this mask to the client’s IP address. For example, if you set the netmask to 255.255.255.0, Squid logs a request from 184.108.40.206 instead of 220.127.116.11. Thus, if someone manages to read the log file, they know only approximately, not exactly, which host (or user) made each request.
If you use log_fqdn, Squid applies the client_netmask before issuing the DNS lookup. For example, Squid will try to find a hostname record for 18.104.22.168 instead of 22.214.171.124.
cache_access_log, useragent_log, referer_log, log_fqdn