In this paper we present an architecture and prototype implementation for discovering key network characteristics, such as hosts, gateways, and topology. The Fremont system uses an extensible set of modules to discover information, based on a variety of different protocols and information sources, rather than a single network management protocol. This approach allows more complete and timely information to be discovered than, for example, using only one protocol, even one as capable as the Simple Network Management Protocol. The discovered information is time-stamped and recorded in a database. The contents of this database are cross-correlated to form an increasingly complete network picture, to direct further discovery, and to highlight inconsistent information.