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 signaling techniques to select compatible Ethernet speeds. Autosensing was developed to make the migration from traditional Ethernet to Fast Ethernet products easier.
What Happens When They Connect
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.