| Port Number | Protocol | Service/Usage |
|---|---|---|
| 20 | TCP | FTP (Data Transfer) |
| 21 | TCP | FTP (Control) |
| 22 | TCP/UDP | SSH (Secure Shell), SFTP, SCP |
| 23 | TCP | Telnet |
| 25 | TCP | SMTP (Simple Mail Transfer Protocol) |
| 53 | TCP/UDP | DNS (Domain Name System) |
| 67 | UDP | DHCP (Server) |
| 68 | UDP | DHCP (Client) |
| 69 | UDP | TFTP (Trivial File Transfer Protocol) |
| 80 | TCP | HTTP (HyperText Transfer Protocol) |
| 110 | TCP | POP3 (Post Office Protocol v3) |
| 111 | TCP/UDP | RPC (Remote Procedure Call) |
| 123 | UDP | NTP (Network Time Protocol) |
| 143 | TCP | IMAP (Internet Message Access Protocol) |
| 161 | UDP | SNMP (Simple Network Management Protocol) |
| 162 | UDP | SNMP (Trap) |
| 443 | TCP | HTTPS (Secure HTTP) |
| 514 | UDP | Syslog |
| 631 | TCP/UDP | CUPS (Common Unix Printing System) |
| 3306 | TCP | MySQL Database |
| 5432 | TCP | PostgreSQL Database |
| 6379 | TCP | Redis (In-Memory Database) |
| 8080 | TCP | Alternative HTTP (Web Servers, Proxies) |
| 8443 | TCP | HTTPS (Alternative, often for admin interfaces) |
| 9000 | TCP | Development (e.g., PHP-FPM, SonarQube) |
You can use the netstat, ss, or lsof commands in Linux to list open ports and their usage dynamically. For example:
netstat -tuln: Lists all listening ports with numerical values.ss -tuln: Similar tonetstatbut faster and more modern.lsof -i :<port>: Checks which process is using a specific port.
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