問65
頻出度★★★
ルータの説明として, 適切なものはどれか。
平成24年度 春期 ITパスポート試験 問65
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 65
Incorrectア
“Performing modulation and demodulation of data” describes a modem. Modulation converts a digital signal into a signal that can be sent over an analog line such as a telephone line, and demodulation is the reverse; the name modem itself comes from modulator and demodulator. A router does not modulate or demodulate signals — it is a device responsible for selecting paths (routing) based on IP addresses — so this is incorrect.
Incorrectイ
“Analyzing the MAC address and forwarding to the destination terminal” describes a switching hub (Layer 2 switch) or a bridge. These are Layer 2 (data link layer) devices of the OSI model; in order to connect terminals within the same LAN efficiently, they look at the destination MAC address of a frame and forward it only to the port that needs it. A router looks not at the MAC address but at the higher-layer IP address, and what it relays is traffic between networks, not traffic between terminals inside a single LAN, so this is incorrect.
Incorrectウ
“Forwarding received data to all terminals” describes a repeater hub. It is a Layer 1 (physical layer) device that amplifies and reshapes the received signal and sends it out unchanged to all of the other ports, so it has no ability to judge the destination and collisions occur easily as traffic increases. A router is a device that analyzes the destination IP address to choose where to forward the data, and it never floods every terminal indiscriminately, so this is incorrect.
- エ
正解 The correct answer is エ. A router is a relay device that operates at Layer 3 (the network layer) of the OSI basic reference model, and its role is to interconnect different networks, such as one LAN with another LAN, or a LAN with a WAN. A router analyzes the destination IP address in the header of the data it receives, compares it against the routing table it holds, and forwards the data to the best next network for reaching that destination (the next router, or a segment beneath it). Only エ satisfies both points: “interconnects LANs and WANs” and “analyzes the IP address and forwards it to the appropriate network”. This is exactly the function that lets a broadband router in a home or an office act as the exit from the internal LAN to the Internet (the WAN side).
ここがポイント
- A router is a network layer (OSI Layer 3) device. It interconnects different networks such as LAN to LAN or LAN to WAN, and uses the destination IP address together with the routing table to choose the network to forward the data to (routing).
- Relay devices are distinguished by “which layer they work at, and what clue they use to forward”. Repeater hub = physical layer (sends the signal to every port); switching hub / bridge = data link layer (MAC address); router = network layer (IP address); modem = modulation and demodulation of signals to match the line.
用語