問49
頻出度★★★
あるネットワークに属するPCが，別のネットワークに属するサーバにデータを送信するとき，経路情報が必要である。PCが送信相手のサーバに対する特定の経路情報をもっていないときの送信先として，ある機器のIPアドレスを設定しておく。この機器の役割を何と呼ぶか。
平成27年度 春期 ITパスポート試験 問49
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 49
- ア
正解 The correct answer is ア (Default Gateway). A PC compares the destination IP address against its own routing table: if the destination is on the same network as itself (the same subnet) it sends the data directly, and if the destination is on another network it forwards the data according to the matching routing information. However, a PC normally does not have individual routes registered for every network in the world. So one default exit is decided in advance, meaning: when nothing matches any routing entry, hand the data over to here for now. The IP address of the device that serves as this exit (usually a router, or the LAN-side interface of a layer 3 switch) is configured on the PC, and that is the default gateway. It matches exactly the role described in the question, namely the device whose IP address is configured in advance as the forwarding destination when no specific routing information is held. The router on the default gateway side takes over the route selection from there, so the PC can reach a server on another network even without knowing detailed routes.
Incorrectイ
A network interface card (NIC) is the hardware (an expansion card or a built-in chip) that connects the PC itself to a LAN cable or to a wireless network. It has a MAC address and is a part on the PC side that serves as its own entrance and exit. It is not the role of another device to which data is handed over when there is no routing information, nor is it the target whose IP address is configured as a destination, so it is wrong.
Incorrectウ
A hub is a concentrator that connects multiple devices within the same LAN. It basically works at the physical / data link layer and passes a received signal on to the other ports as it is (even a switching hub only goes as far as forwarding based on MAC addresses). It has no function to look at IP addresses and relay data to another network, and it is not a device whose IP address is configured as a forwarding destination in the first place, so it is wrong.
Incorrectエ
A firewall is a mechanism that inspects communication at the boundary between the internal network and an external network and blocks traffic that does not match the rules, thereby protecting security. Its role is strictly the control of whether traffic is let through or not, and it does not mean the default forwarding destination for packets that have no routing information. (In real equipment, a router with firewall functions may also serve as the gateway, but as the name of the role asked about in this question it is wrong.)
ここがポイント
- Default gateway = the default forwarding destination to which a packet is handed over when there is no specific routing information for the destination. The IP address of that device is configured in advance on the PC side (it is one of the basic settings, alongside the IP address and the subnet mask).
- Network devices have different roles depending on the layer at which they operate: NIC = the connection part of the machine itself; hub = concentrating connections within the same LAN (physical / data link layer); router (the device acting as the gateway) = relaying to another network based on IP addresses (network layer); firewall = ensuring security by permitting or blocking communication.
用語