問56
頻出度★★★
IPネットワークにおけるルータに関する記述のうち，適切なものはどれか。
平成21年度 春期 ITパスポート試験 問56
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 56
Incorrectア
Mapping IP addresses to domain names is the role of DNS (Domain Name System) and of the DNS servers that provide it. It is the mechanism that performs name resolution, converting a name such as “www.example.co.jp” into an IP address such as “192.0.2.1”. This is not a function of a router, which is a device for relaying packets.
- イ
正解 The correct answer is イ. A router is a device that connects multiple networks and relays packets to their destinations. A router reads the destination IP address attached to the head of a packet, compares it against the routing table it holds, and decides which network the packet should be sent to next. This function of “looking at the destination IP address and choosing a route” is called routing (route selection), which is exactly what イ describes. In terms of the OSI basic reference model it is a network layer (layer 3) device, and it is helpful to remember it in contrast with a switching hub (data link layer, layer 2), which looks at MAC addresses and relays within the same network.
Incorrectウ
Converting between analog signals and digital signals is the role of a modem. It performs modulation, placing the digital signals a computer handles onto an analog transmission line such as a telephone line, and demodulation, the reverse. It is a device that changes the form of the signal; it does not select routes based on IP addresses.
Incorrectエ
Receiving requests from other computers (clients) and carrying out processing or providing data is the role of a server; this describes the server side of a client/server system. Web servers and file servers are examples of this. That role is different from that of a router, which relays packets between networks.
ここがポイント
- A router is a relay device at the network layer (layer 3). Its essential role is to compare the destination IP address against the routing table and select the forwarding route (routing), interconnecting different networks.
- Network devices are distinguished by what they look at when relaying: a router uses IP addresses (layer 3), a switching hub (bridge) uses MAC addresses (layer 2), and a repeater hub simply amplifies and passes the signal along as it is (layer 1). A DNS server performs name resolution and a modem performs analog/digital conversion; neither of them performs the route selection involved in relaying.
用語