Host addresses (127.0.0.1, 0.0.0.0)

Also written: 127.0.0.1, 0.0.0.0

127.0.0.1 is the numeric address behind `localhost`; `fastapi dev` listens only on it, so only your own computer can call the server. 0.0.0.0 means every network address of the machine; `fastapi run` listens on it so other computers can connect.