I guess that is true, but the IP address space today is so large as to be unscannable. Practically speaking, you have to have some evidence that some host is there before you can start prodding at that host.
ZMap is a fast single-packet network scanner optimized for Internet-wide network surveys. On a computer with a gigabit connection, ZMap can scan the entire public IPv4 address space on a single port in under 45 minutes. With a 10gigE connection and PF_RING, ZMap can scan the IPv4 address space in 5 minutes.