From Laffey Computer Imaging:
Mac NetLogger is useful for anyone who runs software or hardware that sends information to a remote syslog (system log). It is similar in this respect to the UNIX syslogd program.
Many routers and other devices can send status info to a syslog. You can keep track of connections and errors with your router using Mac NetLogger. (Provided your router sends out syslog information...)
Mac NetLogger logs to file(s) any information sent to the local ip address on UDP port 514.