令和2年度 ITパスポート試験問78

This question's explanation and your answers so far.

問78

頻出度★★★

通信プロトコルとしてTCP/IPを用いるVPNには, インターネットを使用するインターネットVPNや通信事業者の独自ネットワークを使用するIP-VPNなどがある。インターネットVPNではできないが, IP-VPNではできることはどれか。

出典　令和2年度 ITパスポート試験問78

解説

問 78

Incorrectア
Voice calls using IP phones can be used the same way on both an Internet VPN and an IP-VPN as long as they are TCP/IP based. This is not a feature unique to IP-VPN, so it is incorrect.
イ
正解 The correct answer is イ. An IP-VPN goes through a closed IP network that the telecommunications carrier operates on its own (a dedicated, managed network), so it can guarantee communication quality such as bandwidth and delay through an SLA (a service-level agreement that guarantees quality). An Internet VPN, on the other hand, goes through the public Internet that anyone uses, so its communication quality is best-effort (no guarantee), and bandwidth or quality cannot be guaranteed. Therefore, the thing that can be done with an IP-VPN but not with an Internet VPN is the «guarantee of communication quality such as bandwidth».
Incorrectウ
Prevention of eavesdropping and tampering can also be achieved on an Internet VPN through encryption and tunneling (such as IPsec). In fact, encryption is a feature that matters all the more for an Internet VPN that passes through a public network; it is not unique to IP-VPN, so it is incorrect.
Incorrectエ
Video distribution is ordinary communication that can be carried out on both an Internet VPN and an IP-VPN; it is not something that only one of them can do. It is not unique to IP-VPN, so it is incorrect.

ここがポイント

An IP-VPN uses a telecommunications carrier's closed network, so it can guarantee bandwidth and quality through an SLA (QoS support). An Internet VPN goes through the public Internet and is best-effort, so quality cannot be guaranteed.
Prevention of eavesdropping and tampering through encryption is possible with both methods, so it is not the deciding factor that separates them. Whether or not quality can be guaranteed is the essential difference between an IP-VPN and an Internet VPN.

用語

解答問78イ