ES-IS (End System-to-Intermediate System) is a routing protocol developed by the International Organization for Standardization ( ISO ) as part of their Open Systems Interconnection ( OSI ) model. In the OSI context, an end system (ES) is a node or user device; an intermediate system (IS) is a router .
ES-IS allows a node or user device located in a particular subnetwork to determine the existence and availability of other nodes or user devices on the same subnetwork. ES-IS can also determine the existence and availability of routers that can transmit packets to nodes or user devices on other subnetworks within the same larger network. Working together with the Intermediate System-to-Intermediate System ( IS-IS ) protocol, ES-IS facilitates communication among all the points in a complex network.