The distance-vector protocol designed for use in wide area networks (WANs) is the Border Gateway Protocol (BGP). BGP is an exterior gateway protocol and therefore implemented on border and exterior routers on the Internet. It exchanges information between routers through a Transmission Control Protocol (TCP) session. Routers with BGP implementation determine the shortest path across a network based on a range of factors other than hops. BGP can also be configured by administrators so that certain routes are preferred or avoided. BGP is used by internet service providers (ISPs) and telecommunication companies.