Ports include physical ports and logical ports. Physical ports are interfaces for connecting physical devices, and logical ports are ports for logically distinguishing services. Ports in TCP/IP protocol are logical ports, and different services are distinguished by different logical ports.
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Hosts distinguish different services by "IP address+port number". Servers are usually identified by well-known port numbers. For example, for each TCP/IP implementation, the TCP port number of FTP server is 2 1, the TCP port number of Telnet server is 23, and the UDP port number of TFTP (Simple File Transfer Protocol) server is 69.
To 1992, the well-known port number is between 1 ~ 255. Port numbers between 256 and 1023 are usually occupied by Unix systems to provide some specific Unix services-that is, to provide some services that only Unix systems can provide, while other operating systems may not. IANA manages all port numbers between 1 and 1023.
One difference between Internet extension services and Unix-specific services is Telnet and Rlogin. They all allow you to log on to other hosts through the computer network. Telnet is a TCP/IP standard with port number 23, which can be implemented on almost all operating systems.
Rlogin is only designed for Unix systems (although many non-Unix systems also provide this service), and its famous port number is 5 13. The client usually doesn't care about the port number it uses, just make sure that the port number is unique on the machine. Customer port number is also called temporary port number (that is, short-lived).
This is because it usually exists only when the user runs the client program, while the server runs its service as long as the host is turned on. Most TCP/IP implementations assign port numbers between 1024 and 5000 to temporary ports. Port numbers greater than 5000 are reserved for other servers (services that are not commonly used on the Internet). We can see many examples of assigning port numbers to temporary ports later.
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