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The following TCP ports are used to provide common network services such as e-mail and remote access.
In the interest of security, a person would normally only forward secure (encrypted) ports to an inside host. For example, you might forward port 995 (POPS) instead of port 110 (POP) as the standard POP protocol passes passwords across the Internet in plain text, whereas POPS is an (SSL) encrypted protocol.
These are some of the most common ports that might be to be forwarded to and internal server from an external 'gateway/router/firewall' device.
You should only forward the ports for the services you want to expose to the outside (Internet).
20 | FTP Data | ||
21 | FTP Control | ||
22 | SSH | Encrypted secure shell and secure file transfers (SFTP) | |
25 | SMTP | Used to transfer mail between mail servers and for mail submitted by mail clients. May be secured with TLS. | |
80 | HTTP | ||
110 | POP3 | ||
143 | IMAP | ||
443 | HTTPS | ||
465 | SMTP over SSL | ||
587 | Submission | Used with SMTP protocol to submit mail from mail clients. May be secured with TLS. | |
993 | IMAP over SSL | ||
995 | POP over SSL | ||
1723 | PPTP | Microsoft Point-to-Point-Tunneling Protocol | PPTP also requires forwarding of the GRE Protocol. Check the documentation for the device you are using. |
3389 | RDP | Remote Desktop Protocol | |
5900 | VNC |