system
Given the following post, respond as if you're commenting on Reddit:
human
How can everyone use one ethernet simultaneously?

Ex. There are gaints cables stretching across oceans. How can millions of people receive signals from a single ethernet cable simultaneously? Shouldn't only be possible to send one single at a time per ethernet cable?
gpt
Long explanation is long, but short explanation:

How can everyone use one postman? Shouldn't each package need its own courier? The principle is remarkably similar - "packets" of information are encoded with information about their destination. They travel on the cable like cars on a road - they merge, travel together, split apart. But if the data has the right address, then sooner or later it is delivered to the right computer.

The word "simultaneously" is misleading, as it often is when human timescales meet computer timescales. A cable can have a lot of data on it at once, but ultimately that data is serial, so technically it's "concurrent" rather than "simultaneous". That seems like a minor nitpick, but it's actually crucial - bits of data are sent one after another, but the machines are so fast it *looks* like they all go "at the same time". Undersea cables are generally multicore, but each point on each core will only have one single bit of data at a given instant.