2 1: file transfer server (control connection) (FTP).

23: Remote Terminal Server (TELNET).

25: Simple Mail Transfer Server (SMTP).

80: World Wide Web Server (HTTP).

Extended data:

Precautions:

According to the port number, it can be divided into three categories:

1. Well-known ports: from 0 to 1023, which are closely bound to some services. Usually, the communication of these ports clearly indicates the protocol of the service. For example, port 80 is actually always HTTP traffic.

2. Registration port: from 1024 to 49 15 1. They are loosely bound to some services. In other words, many services are bound to these ports, and these ports are also used for many other purposes. For example, many systems deal with dynamic ports from about 1024.

3. Dynamic and/or dedicated ports: from 49 152 to 65535. Theoretically, these ports should not be assigned to services. In fact, machines usually allocate dynamic ports from 1024. There are exceptions: SUN's RPC port starts at 32768.