Router are network devices that link several computer networks – depending on the perspective – or disconnect. Here, the router analyzes the incoming data packets by their destination address and block them or forwards them. Routed i.e., forwarded, packets arrive then either directly to the router connected to a destination network (including inter-domain) or are forwarded to another router in the network accessible.
Routers operate at layer 3 (network layer / network layer) of the OSI reference model. A router has multiple interfaces (English interface), the networks are reachable. These interfaces can also be virtual. On the arrival of data packets, a router must be the best way to target and thus determine the appropriate interface, via which the data is forwarded. For this he uses a locally available routing table that specifies which port on the router (or the middle) is what the net unreachable. It is also a default route in the routing table must be available. This route shows usually have a higher-order router what is often referred to as standard and default-gateway. But this is no gateway in the sense of the OSI reference model meant but just another router with any more information. Since routing tables are sorted by most systems for the precision, ie first, specific entries, and later less specific, is the default route, as a non-specific, at the end and is used for all targets that do not have more appropriate, and more specific entry in the routing table . have
Some routers also command a so-called Policy Based Routing, taking the routing decision is made not only based on the destination address (Layer 3), but it will also take into account other information, such as the source address, quality requirements or parameters from higher layers like TCP or UDP. Thus, for example, then packages the HTTP (Web) transport, take a different path than packets with SMTP content (mail).
Router, only suitable for routing data packets from 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). Einprotokoll 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).