A common example of a network is the Ethernet networks connecting many of our machines. Ethernet is basically a shared bus using CSMA/CD (Carrier Sense Multiple Access/Collision Detect). That is, each node on an ethernet network can transmit if it doesn't see any other nodes transmiting and it starts by transmitting a distinct signal (alternating zeroes and ones) so it will notice any collisions. This is the same as the bus strategy having multiple arbiters and collision detection.
A computer using Ethernet typically has an I/O device (the Ethernet card) allowing access to the Ethernet. A ``packet'' sent over Ethernet has the following contents.
| Preamble | Dest. | Source | Type | Body | CRC |
| 64 | 48 | 48 | 16 | X | 32 |
The preamble is the series of 64 zeroes and ones to detect collisions. The destination and source fields are 48 bits identifying the parties - each Ethernet card has a unique 48-bit machine address assigned when it is manufactured, though this ``MAC address'' can be changed. The 16 bit type field identifies how the receiver should process the packet. The 32 bit CRC (cyclic redundancy check) at the end gives some quick and simple error checking.
When a packet is sent out on an Ethernet network, it is broadcast so that all nodes on the Ethernet network can see it. Typically, most hosts on the network are not running in ``promiscuous'' mode and ignore packets meant for other nodes. When a node does see a packet meant for itself, it reads the following data, checks the CRC and sends it to the processor. Ethernet suffices to send data between computers on the same Ethernet network but a more involved protocol is necessary for data that travels through multiple networks.