[detaro]

You can't send a packet with a list of destination addresses.

There is multicast, where clients can register "I'd like a copy of all packets to $address please", but it doesn't work on public internet. It's used by some providers for TV streaming inside their networks though.

There's also peer-to-peer approaches, where clients share the data they receive with other clients, similar to Bittorrent.

The most common approach is individual streams from the servers to the clients though.

[tomthehero]

Does youtube use multicast?

[detaro]

No. As I said, multicast generally doesn't work across networks you don't control (and also needs special clients, can't be used with browsers).

[bradknowles]

No.