Ethernet is a hardware and signaling specification developed by Xerox in 1970 for data networking. Ethernet is the de facto networking standard for LAN (local area network) communication and is standardized under the IEEE standard 802.3. Ethernet runs at 10Mb, 100Mb or 1Gb per second. It was originally developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology. Coaxial or twisted pair cable is used.
With over 50 million nodes installed, Ethernet is easily the most dominant data networking technology in use today.
Devices are connected to the network and access it using a 'Carrier Sense Multiple Access' with a Collision Detection (CSMA/CD) protocol (each computer checks to see if another computer is transmitting. If not it starts transmitting. If two computers accidentally transmit at the same time and their messages collide, they both wait and send again in turn. This is the reason that a Ethernet LAN never can work up to it full capacity.
Ethernet networks use unshielded twisted pair (UTP) Category 5 cable. CAT5 cable runs should not exceed 100 meters. Only two pairs of the Cat5 cable is actually used: pair 3 (green) on pins 1 and 2 of a standard RJ-45 connector, and pair 2 (orange) on pins 3 and 6.
EIA/TIA specifies RJ-45 connectors (ISO 8877) for UTP (unshielded twisted pair) cable.

To make sure that no two machines worldwide would end up with the same Ethernet address, each Ethernet controller comes with an address builtin from the factory. In order to make sure that they would never have to reuse addresses, the Ethernet designers allocated 48 bits for the Ethernet address. Ethernet equipment manufacturers have to register with a central authority, to make sure that the numbers don't overlap with any other manufacturer.

pair 1: blue pair: 2 orange pair: 3 green pair: 4 brown (twisted about 3 times per inch) See: Cat5 Cable

Straight Ethernet Cable (CAT5)

Cross-Cable (CAT5)

green LED: Status OFF: port is not active ON: port is active orange/yellow LED: Link/Activity steadily ON: port and the link are active but no link activity blinking ON: port and the link are active and there is link activity