Auto-sensing

- Autosensing is a feature of so-called "10/100" Ethernet hubs, switches, and NICs.
- Compatible Ethernet speeds can be selected using low-level signaling techniques probing the capability of the network.
- 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.

Auto-sensing is an active method of determining link mode. Each interface is expected to transmit specific information in a specific format. If an interface that is expecting to use auto-sensing does not receive this information from the other side, it assumes the other side cannot detect or change its mode.