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Unicast is peer-to-peer. One sender and one receiver. The world wide web is built on unicasting, so is email exchange. Most unicasting is done via TCP, but isn't restricted to this.

A peer to peer network is built on top of unicasting and takes advantage of the fact that you have relatively low-bandwidth uplink nodes distributed among many peers that then "feed" a high-speed consuming node. The receiver then assembles the chopped up pieces into a larger file which is compared to a checksum to make sure that all pieces are genuine.

Contrast this with broadcasting which has one source that "feeds" multiple receivers at the same time (usually on the same LAN).

Similarely multicasting does the same but allows the receiver to "opt-in". Multicasting is designed to be routed and was historically not popular because of the CPU it took up for no charge of the sender. There is a certain turn-around point though when it is costlier to harbour unicast traffic than multicast.