Definition: Network adapters that support both traditional and Fast Ethernet choose the speed at which they run through a procedure called autosensing. Autosensing is a feature of so-called "10/100" Ethernet hubs, switches, and NICs. Autosensing involves probing the capability of the network using low-level signalling techniques to select compatible Ethernet speeds. Autosensing was developed to make the migration from traditional Ethernet to Fast Ethernet products easier.
When first connected, 10/100 devices automatically exchange information with each other to agree on a common speed setting. The devices run at 100 Mbps if the network supports it, otherwise they drop down to 10 Mbps to ensure a "lowest common denominator" of performance. Many hubs and switches are capable of autosensing on a port-by-port basis; in this case, some computers on the network may be communicating at 10 Mbps and others at 100 Mbps. 10/100 products often incorporate two LEDs of different colors to indicate the speed setting that is currently active.
