問55
頻出度★★☆
インターネットのプロトコルで使用されるポート番号の説明として, 適切なものはどれか。
平成21年度 秋期 ITパスポート試験 問55
テクノロジ系ネットワーク › 通信プロトコル
This question's explanation and your answers so far.
解説
問 55
Incorrectア
A unique value held by a physical port that connects to Ethernet describes a MAC address (a 48-bit physical address), not a port number. A MAC address identifies devices within the same network at the data link layer and is not used to distinguish applications.
Incorrectイ
A number indicating the position of a socket (physical port) on a switching hub is merely the sequential numbering of the sockets on the device chassis, which is a different thing from the port number used in protocols. A port number is a logical number carried in the communication data and unrelated to physical sockets.
Incorrectウ
A value that identifies the computer or network device itself when sending and receiving packets describes an IP address. A port number is the value that further determines which application the data is handed to inside the host it reached via the IP address, so the two work at different levels.
- エ
正解 A port number is a 16-bit number carried in the TCP or UDP header, used to identify which of the several applications (services) running at the same time on one computer a communication is addressed to. While an IP address determines which computer, a port number determines which application on that computer. In fact, well-known ports (0-1023) are assigned in advance to representative applications: FTP for file transfer uses 20/21, SMTP for sending e-mail uses 25, and HTTP for the Web uses 80. Therefore エ, which describes it as a value indicating the information gateway for each application, such as file transfer or e-mail, is appropriate.
ここがポイント
- A port number is a logical number used by TCP/UDP to identify an application (service) within a host. Remember them as a pair: IP address = identifies the host, port number = identifies the application.
- Representative well-known ports: HTTP=80, HTTPS=443, SMTP=25, POP3=110, FTP=20/21, DNS=53. Confusing them with physical ports or MAC addresses is a classic trap.
用語