DNS-SD
Many devices are not hard to see. The hard part is knowing what to do after you see them. On a local network, a printer may already resolve by name, but the client still does not know whether it actually offers printing, whether it uses TCP or UDP, what port to use, or whether it advertises extra parameters such as model, paper capabilities, or a path prefix. Knowing only that the machine is called printer.local is not enough to build the connection.