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Routing is the process of selecting best paths in a network. In the past, the term routing also meant forwarding network traffic among networks. However, that latter function is better described as forwarding. Routing is performed for many kinds of networks, including the telephone network (circuit switching), electronic data networks (such as the Internet), and transportation networks. This article is concerned primarily with routing in electronic data networks using packet switching technology. In packet switching networks, routing directs packet forwarding (the transit of logically addressed network packets from their source toward their ultimate destination) through intermediate nodes. Intermediate nodes are typically network hardware devices such as routers, bridges, gateways, firewalls, or switches. General-purpose computers can also forward packets and perform routing, though they are not specialized hardware and may suffer from limited performance. The routing process usually directs forwarding on the basis of routing tables, which maintain a record of the routes to various network destinations. Thus, constructing routing tables, which are held in the router's memory, is very important for efficient routing. Most routing algorithms use only one network path at a time. Multipath routing techniques enable the use of multiple alternative paths. In case of overlapping/equal routes, algorithms consider the following elements to decide which routes to install into the routing table (sorted by priority): #''Prefix-Length'': where longer subnet masks are preferred (independent of whether it is within a routing protocol or over different routing protocol) #''Metric'': where a lower metric/cost is preferred (only valid within one and the same routing protocol) #''Administrative distance'': where a route learned from a more reliable routing protocol is preferred (only valid between different routing protocols) Routing, in a more narrow sense of the term, is often contrasted with bridging in its assumption that network addresses are structured and that similar addresses imply proximity within the network. Structured addresses allow a single routing table entry to represent the route to a group of devices. In large networks, structured addressing (routing, in the narrow sense) outperforms unstructured addressing (bridging). Routing has become the dominant form of addressing on the Internet. Bridging is still widely used within localized environments. ==Delivery semantics== Routing schemes differ in their delivery semantics: * unicast delivers a message to a single specific node * broadcast delivers a message to all nodes in the network * multicast delivers a message to a group of nodes that have expressed interest in receiving the message * anycast delivers a message to anyone out of a group of nodes, typically the one nearest to the source * geocast delivers a message to a geographic area Unicast is the dominant form of message delivery on the Internet. This article focuses on unicast routing algorithms. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「routing」の詳細全文を読む スポンサード リンク
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