問84
頻出度★★★
ネットワークを構成する機器であるルータの機能の説明として, 適切なものはどれか。
平成24年度 秋期 ITパスポート試験 問84
テクノロジ系ネットワーク › ネットワーク方式
This question's explanation and your answers so far.
解説
問 84
Incorrectア
“It connects LAN cables together, amplifies the signal received from one cable, and transmits it to the other cable” describes a repeater. A repeater is a Layer 1 (physical layer) device: it only amplifies and reshapes an electrical signal that has become weak because the cable is long, thereby extending the distance; it has no ability to read IP addresses and choose a route. A repeater hub (hub) with multiple ports works on the same Layer 1 idea, passing a received signal on to all the other ports as it is. Neither relays between networks the way a router does, so this is wrong.
- イ
正解 The correct answer is イ. A router operates at Layer 3 (the network layer) of the OSI basic reference model. It looks at the destination IP address, refers to the routing table it holds, decides which neighboring network the packet should be sent to, and relays it. In other words, the role of a router is to interconnect several networks that have different network addresses — such as a company LAN and the Internet, or a head-office LAN and a branch-office LAN — and to forward packets across the boundary between them. The description “It interconnects different networks and relays packets” is exactly choice イ, and it states the function of a router correctly. It is easier to grasp if you picture a home broadband router linking two separate networks: the provider’s network and the home LAN.
Incorrectウ
“It connects devices within the same LAN and relays packets” describes a switching hub (Layer 2 switch). It is a Layer 2 (data link layer) device that looks at MAC addresses and forwards a frame only to the port of the destination inside the same network. It is similar to a router in that it relays traffic, but the decisive difference is that its scope stays closed inside one and the same network (the same LAN). The essence of a router is that it crosses between different networks, so this description limited to a single LAN is wrong.
Incorrectエ
“It connects a fiber-optic LAN cable and a copper LAN cable, converting the signals on the cables to one another” describes a media converter. It is a Layer 1 device that converts between the signal formats that differ because the transmission media differ (optical signals and electrical signals), so that the two can be joined physically. All it converts is the physical form of the signal; it does not select routes based on IP addresses or relay packets between networks, so it is wrong as a description of the function of a router.
ここがポイント
- Router = a network layer (Layer 3) device. Based on IP addresses and the routing table, it interconnects different networks and relays packets.
- Tell devices apart by layer: Layer 1 = repeater / hub / media converter (amplifying and converting signals), Layer 2 = switching hub (relaying within the same LAN using MAC addresses), Layer 3 = router (relaying between networks using IP addresses).
用語