A sniffer can also be legitimately or illegitimately used to capture data transmitted over a network. A network router reads each packet of data passed to it, determining if it is destined for a destination within the router's own network or if it must be passed further along the Internet. A router with a sniffer, however, may be able to read the data in the packet as well as the source and destination addresses. Sniffers are often used in academic networks to avoid traffic bottlenecks caused by file-sharing applications.
The term "sniffer" is occasionally used for a program that analyzes data other than network traffic. For example, a database could be parsed for certain types of replication.