問72
頻出度★★★
IPネットワークを構成する機器①～④のうち, 受信したパケットの宛先IPアドレスを見て送信先を決定するものだけを全て挙げたものはどれか。 ① L2スイッチ ② L3スイッチ ③ リピータ ④ ルータ
平成30年度 春期 ITパスポート試験 問72
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 72
Incorrectア
The ① L2 switch is a device that forwards at the data link layer (layer 2) based on the MAC address, and it does not look at the destination IP address. The ③ repeater merely amplifies and reshapes the electrical signal at the physical layer (layer 1) to relay it, and performs no address judgment at all. Neither decides the forwarding destination by IP address, so this is wrong.
Incorrectイ
The ④ router decides the forwarding destination by IP address, so it meets the condition; however, the ① L2 switch is a layer-2 device that forwards based on the MAC address and does not look at the destination IP address. Since ① does not meet the condition, this combination is wrong.
Incorrectウ
The ② L3 switch decides the forwarding destination by IP address, so it meets the condition; however, the ③ repeater only relays the signal at the physical layer and does not judge addresses. Since ③ does not meet the condition, this combination is wrong.
- エ
正解 The correct answer is エ (②, ④). The destination IP address is information at the network layer (layer 3) of the OSI reference model, and the devices that use it to decide the forwarding destination are two: ② the L3 switch and ④ the router. A router compares the packet's destination IP address against its routing table, selects the optimal route (the next forwarding destination), and then sends the packet out. An L3 switch is a device that adds routing functionality (layer-3 processing) to an L2 switch, and it can likewise decide the forwarding destination by looking at the IP address. Therefore エ, the combination of ② and ④, is correct.
ここがポイント
- The correspondence between OSI reference model layers and devices: repeater = physical layer; L2 switch / bridge = data link layer (MAC address); router / L3 switch = network layer (IP address).
- The devices that decide the forwarding destination by looking at the destination IP address are the network-layer devices, namely the router and the L3 switch.
用語