問66
頻出度★★★
情報処理技術者試験の日程を確認するために, Web ブラウザのアドレスバーに情報処理技術者試験センターの URL “https://www.jitec.ipa.go.jp/” を入力したところ, 正しく入力しているにもかかわらず, 何度入力しても接続エラーとなってしまった。そこで, あらかじめ調べておいた IP アドレスを使って接続したところ接続できた。 接続エラーの原因として最も疑われるものはどれか。
平成29年度 春期 ITパスポート試験 問66
テクノロジ系ネットワーク › ネットワーク応用
This question's explanation and your answers so far.
解説
問 66
Incorrectア
The role of the DHCP server is to automatically assign network settings such as an IP address to the PC. If DHCP fails, the PC itself cannot obtain an IP address, so it cannot communicate at all, neither by domain name nor by IP address. In this question the connection succeeds by IP address, which shows that DHCP is working normally and is not the cause.
- イ
正解 The decisive point is this: connecting with the domain name 'www.jitec.ipa.go.jp' written in the URL fails, but connecting by directly specifying the corresponding IP address succeeds. Since communication works with the IP address, we can tell that the entire path from the PC to the server (the LAN cable, the IP address assignment, and the router) is all normal. Only one problem remains: the failure of name resolution, which converts the domain name into an IP address. The server responsible for this name resolution is the DNS (Domain Name System) server, and when it fails, even a correctly entered URL cannot have its domain name converted into an IP address, resulting in a connection error. Therefore the most likely cause is a failure of the DNS server, so イ is the correct answer.
Incorrectウ
If the LAN cable were disconnected, the communication path would be physically severed, and you could connect neither by domain name nor even by specifying an IP address. Since the connection succeeds by IP address, the LAN cable is properly connected and normal, so it is not the cause.
Incorrectエ
The default router is the exit toward external networks (the Internet). The examination center to be connected to is an external server, so if the router failed, you could not reach the outside even by specifying an IP address, and the connection would fail. Since the connection succeeds by IP address, the path through the router is normal, so it is not the cause.
ここがポイント
- DNS is the name-resolution mechanism that converts a domain name (e.g., www.jitec.ipa.go.jp) into an IP address. When you cannot connect by domain name but can connect by typing the IP address directly, that is a classic sign of a DNS failure.
- Troubleshooting approach: if communication works by IP address, then the physical wiring, IP address assignment (DHCP), and path (router) are all normal, and the only remaining difference is name resolution (DNS), which lets you pinpoint the cause.
用語