問80
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TCP/IP のポート番号によって識別されるものはどれか。
平成22年度 春期 ITパスポート試験 問80
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 80
Incorrectア
What identifies a LAN card (NIC) installed in a computer is the MAC address (physical address) at the data link layer. It is a 48-bit number assigned to the device when it is manufactured, and it is used to specify the destination of a frame within the same LAN. A port number is a transport layer identifier, and it does not refer to hardware such as a LAN card.
- イ
正解 In TCP/IP, where and to whom data should be delivered is decided by a different identifier at each layer. An IP address gets the data as far as the peer computer (host), but a single computer runs several applications at the same time, such as a web server and a mail server, so another number is needed to decide which of them the arriving data should be handed to. That is the port number at the transport layer (TCP / UDP): a port number identifies the application software (service) running on the peer computer. In practice, well-known port numbers are assigned to the major services: HTTP is 80, HTTPS is 443, SMTP is 25, FTP is 21, and DNS is 53. If a destination is expressed as the pair of IP address plus port number, it uniquely determines which application on which device the communication is meant for. Therefore the correct answer is イ.
Incorrectウ
What identifies the peer computer (host) itself is the IP address at the network layer. A port number is a number used inside the single computer that the IP address has already pinpointed, to distinguish which application the data should be handed to, so it cannot identify the computer itself. Note that a port number alone cannot pinpoint the peer device.
Incorrectエ
A wireless LAN access point is identified by an SSID (ESSID), the identifier used to tell which network you should connect to within the range the radio waves reach, while a MAC address is used as the destination of a frame. Both belong to the wireless LAN world, that is, the physical layer and the data link layer, and have nothing to do with the port number, which belongs to the transport layer of TCP/IP.
ここがポイント
- Organize and memorize which identifier goes with which layer: a MAC address identifies a LAN card (NIC), an IP address identifies a computer (host), and a port number identifies an application (service) on that host. The destination of a communication is uniquely determined by the pair of IP address plus port number.
- Well-known port numbers (0 to 1023) are reserved for the major services. HTTP=80, HTTPS=443, SMTP=25, POP3=110, FTP=21, DNS=53, and SSH=22 come up often, so make sure you know them.
用語