Ethernet:

We can classify network technologies as belonging to one of two basic groups. Local area network (LAN) technologies connect many devices that are relatively close to each other, usually in the same building. The library terminals that display book information would connect over a local area network. Wide area network (WAN) technologies connect a smaller number of devices that can be many kilometers apart. For example, if two libraries at the opposite ends of a city wanted to share their book catalog information, they would most likely make use of a wide area network technology, which could be a dedicated line leased from the local telephone company, intended solely to carry their data.
In comparison to WANs, LANs are faster and more reliable, but improvements in technology continue to blur the line of demarcation. Fiber optic cables have allowed LAN technologies to connect devices tens of kilometers apart, while at the same time greatly improving the speed and reliability of WANs.
Ethernet is a local area technology, with networks traditionally operating within a single building, connecting devices in close proximity. At most, Ethernet devices could have only a few hundred meters of cable between them, making it impractical to connect geographically dispersed locations. Modern advancements have increased these distances considerably, allowing Ethernet networks to span tens of kilometers.
Since a signal on the Ethernet medium reaches every attached node, the destination address is critical to identify the intended recipient of the frame.



For example, in the figure above, when computer B transmits to printer C, computers A and D will still receive and examine the frame. However, when a station first receives a frame, it checks the destination address to see if the frame is intended for itself. If it is not, the station discards the frame without even examining its contents.
The first popular Ethernet medium was a copper coaxial cable known as "thicknet." The maximum length of a thicknet cable was 500 meters. In large building or campus environments, a 500-meter cable could not always reach every network device. A repeater addresses this problem.
Repeaters connect multiple Ethernet segments, listening to each segment and repeating the signal heard on one segment onto every other segment connected to the repeater. By running multiple cables and joining them with repeaters, you can significantly increase your network diameter.