A network tap is a system that monitors events on a local network. A tap is typically a dedicated hardware device, which provides a way to access the data flowing across a computer network. The network tap has (at least) three ports: an A port, a B port, and a monitor port. A tap inserted between A and B passes all traffic (send and receive data streams) through unimpeded in real time, but also copies that same data to its monitor port, enabling a third party to listen. Network taps are commonly used for network intrusion detection systems, VoIP recording, network probes, RMON probes, packet sniffers, and other monitoring and collection devices and software that require access to a network segment. Taps are used in security applications because they are non-obtrusive, are not detectable on the network (having no physical or logical address), can deal with full-duplex and non-shared networks, and will usually pass through or bypass traffic even if the tap stops working or loses power. Terminology The term network tap is analogous to phone tap or vampire tap. Some vendors define TAP as an acronym for test access point or terminal access point; however, those are backronyms. The monitored traffic is sometimes referred to as the pass-through traffic, while the ports that are used for monitoring are the monitor ports. There may also be an aggregation port for full-duplex traffic, wherein the A traffic is aggregated with the B traffic, resulting in one stream of data for monitoring the full-duplex communication. The packets must be aligned into a single stream using a time-of-arrival algorithm. Vendors will tend to use terms in their marketing such as breakout, passive, aggregating, regeneration, bypass, active, inline power, and others; Unfortunately, vendors do not use such terms consistently. Before buying any product it is important to understand the available features, and check with vendors or read the product literature closely to figure out how marketing terms correspond to reality. All of the "vendor terms" are common within the industry, have real definitions and are valuable points of consideration when buying a tap device. A distributed tap is a set of network taps that report to a centralized monitoring system or packet analyzer. Tapping technology methods There are various methods for monitoring a network. Many tapping methods can be used, according to the network technology, the monitoring objective, the resources available and the size