English: Message flows in the presence of a router. On the Internet physical networks are interconnected by routers. Routers forward packets between interconnected networks making it possible for hosts to reach hosts on other physical networks. The message flows between two communicating system A and B in the presence of a router R are illustrated in this diagram. Datagrams are passed from router to router until a router is reached that can deliver the datagram on a physically attached network (called direct delivery). To decide whether a datagram is to be delivered directly or is to be send to a router closer to the destination, a table called the IP routing table is consulted. The table consists of pairs of networkids and the paths to be taken to reach known networks. The path can be an indication that the datagram should be delivered directly or it can be the address of a router known to be closer to the destination. A special entry can specify that a default router is chosen when there are no known paths. Diagram created using: Inkscape 0.47 / KDE 4.4.2 / Kubuntu 10.04