Router is a networking tool that is used to connect to multiple networks. The third level of the OSI (Open Systems Interconnection model) reference tool. Its use different routes (route) to explore the physical layer and they are. In addition, the second-level job as a bridge between different devices is too.

open system reference model ("OSI model"), layer communications and computer network design innovation is an abstract description. The open system development, as was undertaking. The basic network structure as it divided into seven layers, from top to bottom these are in order, application, presentation, session, transport, network, data link, and physical layer. So it often called as "seven fallow OSI model".

Each layer is theoretically similar functions group, who serves up the layer, and receives services from the bottom of the layer. In each layer an "illustration" The top layer provides the service instances, and the layers below to request service. For example, a transport layer over the existing applications to provide impeccable contacts provides the desired path, and to take her from the lower layer to - sought so that the path of the data link layer as possible. Theoretically the two instances in a single layer in each layer to contact the same connection using a horizontal innovation.

Routers can only be suitable for routing data packets of routable protocols such as IP (IPv4 or IPv6) process or IPX / SPX. Other protocols, such as that originally used by MS-DOS and MS-Windows NetBIOS or NetBEUI, which were meant only for small networks and their design is not routable ago, are not forwarded by a router. Packets from these protocol families are usually by systems that work at layer 2, that bridges or switches are processed. Many professional routers can perform when needed, these bridge functions and are then sometimes called Bridge Router. As layer-3 system on the router all the Layer 2 functions, including the broadcast domain. This is particularly important in large local networks to keep the broadcast revenue for the individual stations small. If, however, work-based broadcast services on the routers, then special router needs to receive these broadcasts, evaluate and bring targeted to a different system for processing.

In addition, one-and multi-protocol router (including multi-protocol router to distinguish). A protocol routers are only suitable for a network protocol such as IPv 4 and can therefore be used only in homogeneous environments. Multi-protocol routers dominate the simultaneous use of several protocol families, such as DECnet, IPX / SPX, SNA, IP and others. Today, IP routers dominate the field, as virtually all other network protocols have only a minor role and, if they are still used, can often be encapsulated also (NetBIOS over TCP / IP, IP-encapsulated IPX). Previously, multi-protocol router in larger environments, an essential, then, many manufacturers use different protocol families, so it was absolutely essential that were supported by the router multiple protocol stacks. Multi-protocol router can be found almost exclusively in long-haul or ATM networks.

Important is also the distinction between routed protocols (eg IP or IPX), and routing protocols. Routing protocols are used to manage the routing process and the communication between the routers, for example, so that exchange their routing tables (eg BGP, RIP or OSPF). Routed protocols are the protocols, however, the data packets, which transports the router, underlying (such as IP or IPX).