I see this sort of address used in a bunch of examples. What does it mean exactly? Does it mean it will connect to any/all machines on the subnet that have something listening to that port? Or something else entirely? I see such usage in the docs and in books without explanation. Sort of annoying.
It is explained in the manual.
ZeroMQ supports multiple transports. tcp means you are using the TCP transport.
The address (or endpoint) for the TCP transport has the following format:
tcp://interface:port
When you bind to a local address, interface is either the IP address of a specific interface (network) or *, which means to listen on all interfaces (networks). port is the TCP port or * for a random port.
When you connect to a remote endpoint, interface is the hostname or IP address of the remote machine. port is the TCP port of the remote endpoint.