Within our company network, our office is assigned a particular router with a cluster of IP addresses. Because of some privileges allowed for our group, users who belong to the less-privileged groups within the network are frequently stealing our IP addresses. If the thief's Mac computer has its File Sharing on, I am able to determine who the user is by trying to use the stolen IP address because I get a message saying something like "This IP address is being used by" so and so. However, the thieves have become more clever by having the File Sharing off. Even our IT administrator is unable to trace them and his only recourse is to block the IP address until the thief leaves it. Any thoughts on how to best trace these thieves even if the File Sharing is off is most appreciated. Thank you.