Usually, the port number is not assigned to a specific service or protocol, but is used to identify a specific application or service in network communication. Specifically, the port number is a 16 bit number, which can range from 0 to 65535.
As for the port number, according to the allocation standard of IANA (Internet Assigned Numbers Authority), the port number is generally divided into the following ranges:
- Well known ports: 0 to 1023, used for some common services or protocols, such as HTTP (80), HTTPS (443), FTP (20, 21), SSH (22), Telnet (23), SMTP (25), etc.
- Registered ports: 1024 to 49151. These ports can be customized by users or applications, but may be limited by some conventions.
- Dynamic or Private Ports: 49152 to 65535. These ports are usually used for temporary or private connections, such as temporary ports dynamically assigned to client applications.
In this case, the standard port used by SMTP (Simple Mail Transfer Protocol) is 25. SMTP is a protocol for e-mail transmission, which sends mail between e-mail servers. The e-mail client uses SMTP to send mail, while other protocols such as POP3 or IMAP are used to receive mail.
It should be noted that in some cases, there may be other non-standard usages or services that use specific port numbers, depending on the network environment and application configuration. Therefore, the actual use of port 25 may vary depending on the environment.
