1. A method, comprising: monitoring routable traffic through a border router between a local computer network and a global computer network; characterizing use of one or more global prefixes of the traffic; determining a number of global prefixes that can be mapped into context identifiers (IDs) having a shorter bit-length than the global prefixes; determining, up to the number, a particular set of the global prefixes that are most frequently used; mapping the context IDs to the particular set of global prefixes; distributing the context IDs into the local computer network; and converting between the context IDs and the global prefixes at the border router.