Ethernet is one of many layer 2 technologies. Others include: Token Ring, FDDI, and Gigabit Ethernet. Layer 2 can be considered as a connection between the physical layer and the network layer.
Ethernet works on a principle called "Carrier Sense Multiple Access / Collision Detect" or "CSMA/CD". "Multiple Access" refers to the way multiple computers access a single Ethernet segment. Originally, this meant a single coax segment. Now this refers to all stations on the same "side" of a network switch or bridge. "Carrier Sense" means that each station on an Ethernet segment can determine if any other station is transmitting. This way, if one station is transmitting, another won't begin. However, if two stations begin transmitting at roughly the same time, carrier sense will not prevent the two transmissions from interfering with each other. This is known as a collision. The "Collsion Detect" feature of Ethernet will allow a station that is transmitting to detect when another station is also trasnmitting (and a collision has occured), and it will then send a jamming signal to all stations on the Ethernet segment. When this signal is received, each station will cease transmission, and will begin a randomized countdown to determine when it can transmit again.
Each Ethernet station has a unique, burned-in hardware address
known as a Medium Access Control (MAC) address. This 48-bit
address is used for all Ethernet communication at layer 2. MAC
addresses are usually represented in hexadecimal, two hex
characters (one byte) at a time. An example hex address:
00-20-AF-F7-B9-1C
The first 3 bytes represent the manufacturer, in the above example,
00-20-AF is indicator of 3Com Corporation. This leaves 3 bytes
(24 bits) for the manufacturer to use, meaning for each assigned
manufacturer code, there are 16,777,216 (2^24) addresses that may
be assigned.
No matter what higher layer protocols are being used, all addressing in Ethernet must be eventually done at layer 2 as a MAC. For example, if TCP/IP is being used at layers 3 and 4, each computer will be assigned a 32-bit IP address. Before communicating, an Ethernet-attached station must resolve any IP address to a MAC address. This is done using Address Resolution Protocol, or ARP.
Ethernet data is transmitted in bundles known as frames. All Ethernet frames follow the same format, shown below:
The destination MAC address is the intended receiver's address. This address can be a specific computer's MAC address, or if the frame is to be broadcast to all stations on the Ethernet segment, an address of all one's is used (or FF-FF-FF-FF-FF-FF in hex). The type field indicates how the data field should be interpreted. For example, type 0800 hex is used for IP.