Ethernet is a technology to distribute information over cable, radio or light. The technology is primary designated for Local area networks but is increasingly used for long distance links by telecom operators. The reason for this is the common use of the technology that makes it inexpensive to produce and purchase.

The speed on Ethernet has raised over time. It started at 10 Mbit (shared media) and continue to 100, 1000 and 10 000 Megabit (10 Gbit). Today the common standard on Ethernet card is 1 Gigabit.

Just like IP Ethernet has a global addressing system called MAC addresses (Media access control) where every card is delivered with a unique address, commonly burned in on the circuits in the equipment. Every vendor can apply for their own series of addresses so you can tell the vendor of the card from the address.

To connect different Ethernet based equipment so called hubs or switches is used. Hubs have disappeared in the last few years as it uses a common media and all ports share the same bandwidth. In a switch traffic passes over unique paths between to ports which means you can utilize full bandwidth on two different pairs of ports.

An ethernet card represent the link layer (layer 2 in OSI) and has to be connected to other ethernet cards on the same media and local area network. If you want to communicate with another media you need to traverse a level 3 unit like a router or a layer 3 switch. So the Mac addressing system is only used locally, for global communication IP addresses is used.

When an Ethernet card want to communicate with another unit it sends a message to a special range of addresses that is dedicated for broadcast messages. The equipment with the higher level address then see the message and answer direct to the sender. Because of this mechanism the Ethernet protocol is not very scalable but practically limited to a couple of hundreds of machines. Over that it becomes slow and very vulnerable.This is a good reason to use routers locally.

Messages that are sent outside the local area network is always sent to the local router (default gateway). The task of this router is to distribute the message further on to the next router (see Internet)