Any device that captures and interprets the network traffic between two or more connected computer systems. The traffic can then be decoded so that it is possible to see what processes are occurring.

By examining the flow of traffic, protocol analysers can be used to find out where problems (such as bottlenecks or the failure of a network device) are on a LAN.

Advanced protocol analysers can also provide statistics on the traffic that can help to identify trends that may in future lead to further problems with the network.