How do IP routers build and maintain their forwarding tables?

Ethernet bridges always have the option of fallback-to-flooding for unknown destinations, so they can afford to build their forwarding tables “incrementally”, putting a host into the forwarding table only when that host is first seen as a sender. For IP, there is no fallback delivery mechanism: forwarding tables must be built before delivery can succeed. While manual table construction is possible, it is not practical.