ネットワーク管理者は、新しいブロードバンド回線の設置を進めています。管理者は、/30 プレフィックスを付けて新しい静的 IP アドレスを割り当てます。ただし、管理者はインターネットに接続できません。次の情報があるとします。
IPアドレス:4.11.17.6
サブネットマスク: 255.255.255.252
デフォルトゲートウェイ: 4.11.17.1
DNS1: 1.1.1.1
DNS2: 8.8.8.8
最も考えられる原因は次のうちどれですか?
正解:D
The most likely cause of the administrator's inability to connect to the internet is the incorrect default gateway.
The default gateway is the IP address of the router that connects the local network to the internet. The default gateway should be in the same subnet as the IP address of the device. However, in this case, the IP address of the device is 4.11.17.6 and the subnet mask is 255.255.255.252, which means the subnet has only four addresses: 4.11.17.4, 4.11.17.5, 4.11.17.6, and 4.11.17.7. The first and the last addresses are reserved for the network and the broadcast, respectively, so the only valid addresses for the device and the gateway are
4.11.17.5 and 4.11.17.6. Therefore, the default gateway should be 4.11.17.5, not 4.11.17.1, which is in a different subnet. The routing table, the subnet mask, and the DNS are not the causes of the problem, as they are either correct or irrelevant for the internet connectivity.
References
* 1: CompTIA Network+ N10-008 Certification Study Guide, page 83-84
* 2: CompTIA Network+ N10-008 Exam Subnetting Quiz, question 8
* 3: CompTIA Network+ N10-008 Certification Practice Test, question 3