Most networks use broadcasting technology (view info), which means that each packet that a device transmits over the network can be read by any other device connected to the network.
WiFi networks and networks with a HUB device use this approach, however smarted devices such as routers and switches will only route and pass packets to the machines available in their route table. More information about broadcast networks can be found at https://en.wikipedia.org/wiki/Broadcasting_(networking)
In practice, all computers except the recipient of the message will realize that the message is not intended for them and ignore it. However, many computers can be programmed to see each message that crosses the network.
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