What is the use of scanned ports?

A lot of users ask the most questions is: I use scanning software to sweep a machine on a number of open ports, but those ports in the end what is the use of Ah, today, I use a real scan to tell you the use of the port swept to you.

Swept host: 192.xxx.xx.x

Host IP number: 4

Security Vulnerabilities found: 7

Security Weaknesses: 45

System: Standard:solaris 2.x, Linux 2.1.???, Linux 2.2, MacOS

Telnet (23/TCP)
SSH (22/TCP)
FTP (21/TCP) (Discovery of security vulnerabilities)
Netstat (15/tcp)
Daytime (13/TCP)
Systat (11/TCP)
Echo (7/TCP)
Time (37/TCP)
SMTP (25/TCP)
www (80/tcp) (Discovery of security vulnerabilities)
Finger (79/tcp)
Auth (113/tcp)
SUNRPC (111/TCP)
Pop-2 (109/TCP)
Linuxconf (98/TCP)
IMAP2 (143/TCP)
Printer (515/TCP)
Shell (514/TCP)
Login (513/TCP)
EXEC (512/TCP)
Unknown (693/TCP)
Unknown (698/TCP)
Unknown (727/TCP)
SWAT (910/TCP)
Unknown (1025/TCP)
Unknown (1039/TCP)
Unknown (1038/TCP)
Unknown (1037/TCP)
Unknown (1035/TCP)
Unknown (1034/TCP)
Unknown (3001/TCP)
Unknown (6000/TCP)
Echo (7/UDP)
General/tcp
Daytime (13/UDP)
Unknown (728/UDP) (Discovery of security vulnerabilities)
Unknown (2049/UDP)
Unknown (681/UDP)
Unknown (2049/TCP) (Discovery of security vulnerabilities)

Ports that can be logged in with Telnet (23/TCP)

This information indicates that the Telnet service is running, where you can log on remotely to the host, which is dangerous if you can log on anonymously, and anyone can send data between the server and the client.

Vulnerable vulnerabilities found (21/TCP)

Where I found a directory is writable:/incoming

FTP Port (21/TCP)

The FTP Service Telnet service can be logged on anonymously, and on some machines it allows you to execute remote commands, such as CWD ~xxxx, if you can CWD root success, you can get the highest privileges, but such a good thing seems to be few. In addition, sometimes you can use it to get an available account (guest), or to know what system the host is running

13/TCP (Daytime)

From here you can see that the server is running 24x7, which helps an intruder have enough time to get the system that the host is running, plus UDP is running 24x7, which allows intruders to cheat on the host's denial of service through UDP spoofing.