We describe a new network service called speccast. Speccast offers a generalized addressing and routing abstraction on which a rich variety of semantic services can be built, and, as such, provides a vehicle for studying the relationships among routing, addressing and topology. Unlike overlay-based systems, we study a more basic problem, in which the topology of the network is given, and there is not necessarily any pre-existing underlying network service. In the speccast model, each packet carries a destination predicate and the network's job is to deliver the packet to all nodes satisfying that predicate. After showing how this generalized routing service subsumes other services both traditional (unicast, multicast) and emerging (publish-subscribe), we present a layered solution for a specific class of simple predicates. We examine the tradeoffs in various forms of our approach, and compare it to existing solutions for unicast and multicast. Studies using transit-stub graphs show that our generic service performs comparably to existing solutions for traditional services, while also effectively supporting new and emerging services.