Physical layer

The first Ethernet networks, 10BASE5, used thick yellow cable with vampire taps as a shared medium (using CSMA/CD). Later, 10BASE2 Ethernet used thinner coaxial cable (with BNC connectors) as the shared CSMA/CD medium. The later StarLAN 1BASE5 and 10BASE-T used twisted pair connected to Ethernet hubs with 8P8C modular connectors (not to be confused with FCC's RJ45).

Currently Ethernet has many varieties that vary both in speed and physical medium used. Perhaps the most common forms used are 10BASE-T, 100BASE-TX, and 1000BASE-T. All three utilize twisted pair cables and 8P8C modular connectors (often called RJ45). They run at 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s, respectively. However each version has become steadily more selective about the cable it runs on and some installers have avoided 1000BASE-T for everything except short connections to servers.

Fiber optic variants of Ethernet are commonly seen connecting buildings or network cabinets in different parts of a building but are rarely seen connected to end systems for cost reasons. Their advantages lie in performance, electrical isolation and distance, up to tens of kilometers with some versions. Fiber versions of a new speed almost invariably come out before copper. 10 gigabit Ethernet is becoming more popular in both enterprise and carrier networks, with development starting on 100G Ethernet.

Through Ethernet's history there have also been RF versions of Ethernet, both wireline and wireless. The currently recommended RF wireless networking standards, 802.11 and 802.16, are not Ethernet, in that they do not use the Ethernet link-layer header, and use control and management packet types that don't exist in Ethernet – it would not be simply a matter of modulation to transmit Ethernet packets on an 802.11 or 802.16 network, or to transmit 802.11 or 802.16 packets on an Ethernet network.